Cellulose acetate phthalate (CAP) is a commonly used enteric coating polymer. CAP powder has been studied by various methods to determine characteristics that have an influence on its functionality. While some of the parameters are well known, such as free-acid content and substituent composition, new methods have been developed to examine them. Other characteristics, such as the molecular mass distribution, have not been reported earlier. Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and thermal analysis have also been performed on fresh samples, as well as samples stored under various temperature and humidity conditions. Humidity is by far a more critical storage parameter than temperature, although the two act in conjunction; high humidity is more deleterious to the functionality of the polymer than high temperature. Functionality in this case is taken to be determined by the substituents and by the molecular mass distribution. Mass-average molecular mass of a number of batches of the polymer has been measured and ranges around 48 kg/mol with a degree of polydispersity of 1.6. A method to perform a rough estimation of the molecular mass of CAP has also been suggested based on knowledge of the substituent content. It may be possible to use the values of <Mn> and <Mm> obtained here for any other batch of the same viscosity grade of CAP. NMR has been employed to determine the fraction substituents in the polymer. However, an attempt to obtain the pattern of substitution of the CAP molecule by NMR was unsuccessful. Glass transition temperatures of CAP samples were measured. However, this characteristic of the polymer is judged not as sensitive to the loss of substituents as the molecular mass. Thermal treatment of the polymer in oxygen and inert atmospheres gave slightly different degradation products.