Solid-state NMR and thermal analysis techniques were used to compare the mobility of poly(vinyl butyral) (PVB) in the presence and absence of both alumina filler particles and plasticizer. The relative mobility of main-chain and side-chain carbons increases in the presence of both plasticizer and alumina as seen from an increase in the average cross-polarization rates, 〈TCH〉, and from a general decrease in the rotating frame spin-lattice relaxation rates, CT1ρ. Unlike the main-chain carbons, inversion recovery cross-polarization (IRCP) data on solution cast composites show that the side-chain methyl groups in neat PVB samples are best characterized by a monoexponential cross-polarization model. However, in the presence of alumina, a fraction of the methyl groups becomes less mobile as indicated by the biexponential IRCP behavior. Polymer phase mobility also increases below the critical pigment volume concentration (CPVC) as seen from decreasing transition temperatures using dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). This is accompanied by an increase in the rigidity of a portion of the polymer phase above the CPVC as indicated by an increase in the apparent Tg, along with an increase in the mobility of another portion as indicated by the appearance of a low-temperature DMA transition. These trends are consistent with an increase in polymer chain packing heterogeneity in the presence of alumina. © 1993 John Wiley & Sons, Inc.