The article discusses the concept of “decadence” and its derivatives (decadent, decadisme, decadentism) in the French literature in 1885 and 1886, when it underwent a certain “institutionalization”, denoting since that time a literary movement as well. The author analyses program writings, like “ ‘Les déliquescences’, poèmes décadents d’Adoré Floupette”, Jules Laforgue’ “Complaintes”, the activities of the littérature revues “La Revue Wagnérienne”, “Le Décadent littéraire et artistique”, “La décadence. Littéraire et artistiques”, “Le Symboliste”, the theoretical publications of Paul Bourget, Paul Bourde, Anatole Baju, René Ghil, Jean Moréas etc., poetry of S. Mallarmé, P. Verlaine, A. Rimbaud etc. The paper emphasizes the role of irony as an integral part of the poetics of decadence in these years, the relationship, and sometimes “coincidences” with symbolism, the institutionalization of which in France was largely provoked by decadence.