Compatibility of cellulose acetate hydrogen phthalate (CAP) and poly methyl methacrylate (PMMA) has been investigated by solution viscometric, ultrasonic and differential scanning calorimetric (DSC) methods. From viscosity measurements, Krigbaum and Wall polymer-polymer interaction parameter is evaluated. Ultrasonic velocity and adiabatic compressibilites versus blend composition are plotted, and found to be linear. T-g values are calculated using Gordon-Taylor and Fox equations. The results obtained reveal that CAP forms a miscible blend with PMMA in the entire composition range. Compatibility may be due to the formation of hydrogen bond between the carbonyl group of PMMA and the free hydroxyl group of CAP. Compatibility is further predicted from dielectric measurements.