In August 1919 he wrote to Albert Biolley with news of some progress on a Flute Concertino: «Hier encore j'ai ebauché un Tutti et un début... la thème de l'Andante a suivi naturellement. Mais en ce moment je ne sais pas où mettre la tête.» What became of these sketches is uncertain; it could be that they were incorporated in the score of Doktor Faust. Yet, at this time the opera was stagnating while Busoni practised the piano for his English tour. Nevertheless, towards the end of the 1919-20 concert season he set to work and composed the Divertimento for flute 'as easily as writing a letter'. The date at the end of the score is 24 May 1920. On that day he also wrote to Biolley:

Je viens de terminer le «Divertissement pour Flûte et Orchestre» (manquent encore trols notes à la 2de trompette) et je me sens content. - Cest un 'pendant' au «Concertino de Clarinette»; plus fantastique peut-être, peut-être aussi plus viril. Et une idée plus court (probablement par l'idée qui m'a manquée).

The Divertimento opens with a generously proportioned tutti. Although the language is unmistakably Mozartian, especially in rhythmical matters, Busoni's use of tonality is as paradoxical as ever. The firm ground of B flat major, which he establishes in the opening phrases, is slowly eliminated and, after a few bars, the work floats from one key-centre to another as if defying the laws of gravity, with only the briefest references to the tonic. At the end of the tutti a D major fanfare is passed down the line from first trumpet (naturale) to second trumpet (con sordino) and eventually to the solo flute, which makes its first entry disguised, as it were, as a third trumpet - a delightful touch of Busonian humour. The flute then demonstrates its superiority over the trumpets by running the fanfare over the full range of the instrument and from now on dominates at all points. Its varied statement of the opening theme is a good example of the major-minor dialectic. [...]
A second theme of a more plaintive nature appears, and the material is handled much in the manner of classical sonata form. Yet Busoni runs the same danger in this music as he had experienced as librettist in Arlecchino - that of overcompression. His tendency to conciseness is taken so far in the Divertimento that a logical unfolding of the material is no longer apparent, except perhaps to a musician with an abnormally rapid mind. [...]
In the Divertimento, this speed of thought makes the listener's task dizzy but stimulating, yet the classical landscape of the piece is sufficiently familiar to prevent him losing his bearings altogether. The sonata Allegro with which the work opens is followed by an Andante sostenuto, a literal rearrangement of the Elegy for Clarinet (without its coda), lightly scored for pizzicato strings and occasionally giving the orchestral clarinet the lead, as if to remind the listener of the music's rightful owner. At the end of the long theme Busoni introduces a reminiscence of the second subject of the opening Allegro, effecting a transition to the closing section. This begins as a literal recapitulation, but after eight bars the main theme is thrown away in favour of a fleeting chromatic tarantella. The flute increases the pace with a long and dazzling run, punctuated by a syncopated figure in the orchestra [...]
In the First Tableau of «Doktor Faust» Busoni uses this phrase just once, to depict laughter. He was probably influenced by the Act I finale of «Così fan tutte», an opera with which he had only recently become acquainted. [...]
On 21 August 1920, Busoni's staunchest friends in Zurich, including Volkmar Andreae, Philipp Jarnach, Albert Biolley and the members of the Torthalle Orchestra, were present at a small and unceremonious farewell - a trial rehearsal of the Divertimento. On 9 September Busoni left Zurich for ever and returned to Berlin, 'the city of darkness' as Dent so aptly described it. He took with him 1,000 new books which he had acquired during the war, the first half of the full score of «Doktor Faust» and the sketches for his next work, the «Toccata» for piano. He left his wife and children temporarily in Zurich; to his beloved Giotto and many Zurich friends he bade a final farewell. He wrote:

In Berlin I shall find discord and deprivation, but also satisfaction and diversity of interests... I would in any case have departed from Zurich, which has returned to a state of stupor; the people of Switzerland will have to find their miracles among their own ranks. [Letter to Marchese di Casanova, 7.8.1920] [BEAUMONT, pp. 266-268]