Equations for use in studies of dissolution are derived from simple Noyes-Whitney kinetics. In one model the amount of drug added to the test system, Wo, is equal to the saturation capacity of the system, Ws In the second model an equation is derived for use when Wo ≠ Ws It was found that the dissolution of hydrocortisone, in both distilled water and nonionic surfactant solutions, obeyed Model I equations up to about 40% saturation. The results obtained using Model II equations showed considerable variation in the dissolution rate constant. The effect of low concentrations of an n-alkyl polyoxyethylene surfactant upon the dissolution of hydrocortisone was investigated. It is shown that the plot of the dissolution rate constant against the surfactant concentration shows a pronounced maxima at the region of the CMC. Possible reasons for this finding are discussed.