AbstractMixed monolayers of pheophytin a and L‐α‐dimyristoyllecithin (DML) are investigated on the water surface. The studies gain their special value from the simultaneous measurement of surface pressure and fluorescence intensity as a function of the covered area per molecule. A phase separation in the liquid state of the mono‐layer is established. Phase (1) exists almost exclusively of pheophytin molecules. Phase (2) exists essentially of DML domains solubilizing pheophytin in a concentration of 15 mol‐%. During the DML main transition the pheophytin solubility decreases to about 2 mol‐%, the excess pheophytin being precipitated within the surface layer. During the pheophytin main transition an ordered structure below the surface layer is formed. A stabilizing interaction between the pheophytin domains and their environment is observed and discussed. A sharp fluorescence change at a pressure below 0.5 dyn/cm indicates another phase transition. It very probably involves an unwrapping of pheophytin from water molecules.