Liberation of active agent from suspensions containing sulfadimidine was investigated. There are three physico-chemical steps of the process: (i) dissolving active agent in the vehicle of suspension, (ii) partitioning dissolved molecules between the membrane and the suspension, and (iii) diffusion of active substance through membrane into the acceptor phase. The vehicle of suspensions contained surface active agents in 0.01 to 1.00 mass % concentration range. The liberation of active substance was examined with Hanson vertical diffusion cell. A multiplicative function, a square root one and an exponential equation were fitted at the analysis of results. It was established that the square root function and the exponential one fitted well to the measuring points. Relationships were found between the concentration and HLB values of surfactants and the rate constant of liberation. There is a characteristic correlation between the constants of Higuchi's equation and modified Weibull one, too.