On the basis of a significant number of unpublished documents, here published for the first time, the article reconstructs the historical and scientific origins of Lavoisier's Mémoires de physique et de chimie. Because of the paucity of primary sources available so far, this work has previously received little attention and its 'publication' is commonly attributed to Madame Lavoisier's effort to revive the memory of her husband. In contrast with this image, this article suggests that Madame Lavoisier had only a subsidiary (...) role and that Armand Séguin's contribution to both the editing and the scientific organization of the Mémoires needs a historical reassessment. Lavoisier's collaboration with Séguin, dating from 1792, resulted in a work the aim of which was far more ambitious than that of collecting already published memoirs. Furthermore, it is argued that the contents of the first volume of the Mémoires indicate an important evolution in Lavoisier's chemical theory. (shrink)