The kinetics of UV polymerization of two amphiphilic diacetylenes CH3(CH2)11C=CC=(CH2)8COOH (Cd-salt) and [GRAPHICS] has been studied in situ in layers synthesized according to the Langmuir-Blodgett method. Three consequent stages are observed during continuous irradiation : polymerization, photochromic transition and photolysis. The changes in the refractive index, the adsorption constant and the film thickness have been measured for each stage. The photopolymerization of diacetylenes follows a multistage route in a multilayer system. Spontaneous polymerization under photoinitiation has been observed for the films consisting of less than six layers. Models are discussed giving a qualitative description of the experimental features of the process.