A theoretical method was developed to describe the time course of the available surface area (S (t)) in the dissolution process of solid dosage forms. In general, dissolution of solid dosage forms includes disintegration and deaggregation (dispersion of disintegrated granules and powders), and since the mechanism is very complex, S (t) in the dissolution medium cannot be determined by numerical calculation. However, the integrated value of S (t), which can be considered to be the available surface area generated during the dissolution time course, can be calculated from the dissolved amount and the dissolution parameters. In addition, the ratio of the available surface area generated by time t to the total available surface generated can be regarded as a cumulative probability distribution. By the regression of this ratio (F(t)) to the probability distribution, the time course of S (t) was able to be determined numerically. The application of this theory to the dissolution of flufenamic acid in tablets is described.