Diffusion coefficient (D) shows a weak negative correlation to molecular weight in both systems (A,D). In the reservoir system (A,B,C), the combined empirical parameter Dk [M = ADkCSt] is inversely correlated to API solubility (aq, 25°C) and proportional to the partition coefficient “P” as illustrated by the logarithmic correlation between LogP and Dk (B). D and k can not be independently determined from reservoir system release data. In the matrix system, described by the empirical equations Y’ shows similar yet opposite correlations to Dk in the reservoir system in that it is inversely correlated to drug solubility (F) and negatively proportional to the partition coefficient “P” (E).