An aqueous polymer coating composition containing at least one latex or pseudolatex is coated on a substrate in a high humidity coating process followed by heat treatment of the coated substrate above the film forming temperature of the coating at low humidity. The coated substrate gives a stable reproducible dissolution profile substantially insensitive to temperature or humidity conditions upon storage. The high humidity coating process is achieved by addition of water to the coating chamber through dilution of the coating formulation or humidification of inlet air. Contrary to the conventional low humidity coating process, residual water is retained in the coating layer of the coated substrates in the coating step. When the coated substrates are heat treated at a temperature greater than the minimum film forming temperature, the residual water in the coating layer will ensure adequate capillary force for the completion of film coalescence. High humidity heat treatment is not needed.