Light scattering by stearic acid film on the surface of liquid (water) was studied for the wavelength 632,8 nm (He-Ne laser). It was found that microparticles present on the liquid surface were the collapse centres in the monolayer of the substance under study when its surface pressure was increased. Thus various microparticles (dust, contaminations etc.) on the liquid surface give rise to substantial decline in quality of the Langmuir - Blodgett films when they are transferred onto the solid substrate.