Aqueous latex dispersions of ethylcellulose are often used to form controlled release coatings on pharmaceutical dosage forms. Occasionally, these products exhibit pH-dependent release characteristics, with release rates typically being slow in water and dilute acid solutions and faster in solutions buffered to pH values near neutral. The source of this pH-dependent release is not obvious, since the principal mechanism of release is by osmotic pumping, the coating polymer is neutral and the effect is seen even with drugs that do not ionize within the pH range of interest. In this research, we used phenylpropanolamine HCl pellets overcoated with ethylcellulose to investigate the source of pH dependency of release. pH dependency of release was observed in all batches plasticized with dibutylsebecate and column dried to an end-product temperature of 43-45°C. However, when batches were heated for an additional 2 h in a forced air oven, or triethylcitrate was used as the plasticizer, release was virtually independent of pH. It was observed that heating the product above the Tg of the plasticized film was associated with more consistent release profiles. Contact angle measurements suggested that these effects were associated with changes in surface behavior upon heating. Although titration experiments demonstrated batch to batch variation in residual group content of ethylcellulose, little change in the titration data was seen after heating the ethylcellulose at 60°C for 2 h. On the basis of these data, we hypothesize that heating the product above the Tg of the plasticized film results in film relaxation, enabling even distribution of the surfactant throughout the film and thus minimizing the surface effects that are associated with pH-dependent release.