There are strong seasonal variations in the depth of the thermocline and in the sea surface temperature in the western part of the Gulf of Finland. A numerical model, developed by Kraus and Turner, was used to calculate the depth of the thermocline and the temperature of the mixed layer from June to October in 1969 and 1974. Observed daily mean values of total incoming radiation, air temperature, wind velocity, air humidity and cloudiness were used as input data from which the thermal energy balance on the sea surface was calculated. The model is sensitive to the drag coefficient of the wind and to the absorption coefficient of the solar radiation for which the values cD =0.8.10−3 and ß=0.25 m−1 were used in this work. For the mixed layer temperature the results of the model are 2-4° C too small. The depth of the thermocline can be predicted with an accuracy of ±5 meters.