This article explores the language of the promise in Derrida’s ‘Avances’ and Mémoires pour Paul de Man and in Proust’s À la recherche du temps perdu. Derrida considers the promise to be impossible and necessary, tenable and untenable, finite and infinite. These Derridean stipulations are used to read the imagery and structure of promises in Proust’s novel. While the promise in À la recherche punctuates the lives of couples unreliably, it is also shown to be connected to epiphanic moments intermittently granted by nature and art, and often signalled by the figure of the rainbow. Special attention is paid to Vinteuil’s septet as a promise, and to connected semantic fields within this episode. Finally, the Derridean promise of writing is proposed as a means of rereading Le Temps retrouvé, and the promise as compromise is suggested to be a useful intermediary between the two opposing critical views (circular versus open-ended) surrounding the close of À la recherche and the complex question of the relationship between the book we have just read and the book the protagonist plans to write.