Miscibility characteristics of cellulose acetate hydrogen phthalate (CAP) and poly(vinyl pyrollidone) (PVP) have been investigated by solution viscometric, ultrasonic, and differential scanning calorimetric (DSC) methods. From viscosity measurements, Krigbaum and Wall polymer-polymer interaction parameter Delta b was evaluated. Ultrasonic velocity and adiabatic compressibility have been plotted versus blend composition and are found to be linear. Variation of T-g with composition follows Gordon-Taylor equation. T-g values have also been calculated from the Fox equation. The results obtained reveal that CAP forms a miscible blend with PVP in the entire composition range. Compatibility may be due to the formation of hydrogen bonding between the carbonyl group of PVP and the free-hydroxyl group of CAP. Compatibility has also been confirmed from dielectric measurements. (C) 2000 John Wiley & Sons, Inc.