Once the preliminaries of peace had been signed in January 1783, after the war of American independence, exchanges between British and French men of science resumed their normal course. On a visit to Paris in 1783, the francophile Charles Blagden (with the encouragement of Joseph Banks) made a number of contacts that fostered relations between the Royal Society and the Académie royale des sciences. In the course of this and several subsequent visits to France, Blagden became especially intimate with the chemist Claude-Louis Berthollet. His correspondence, now in the Royal Society, is a rich source for our understanding of some of the leading scientific debates of the day, in particular concerning the nature of water, which forms the main subject of this article.