The aim of this article is to report on the relationship between Liszt and Fétis via their letters, their articles and Fétis’ theoretical works and notes. Evidence of their relationship spread over 40 years shows different junctures (and disjunctures) between them. Reviews, organizations of concerts, advice on their respective opuses, discussions on musical theories or on ancient and religious music… they share quite a varied level of interest. The article envisages the various aspects of their relationship. The exchanges between the two men concerned by the main questions of their time (often as avant-garde thinkers) shows how rich can be the collaboration between a musicologist and a composer.