Melting behavior under high pressure of nylon 6, nylon 6-clay hybrid (NCH) and poly(butylene terephthalate) was investigated by high-pressure differential thermal analysis (DTA). It was found that the melting temperature and the pressure dependency of the melting temperature of NCH were low compared with those of nylon 6. Using the melting temperature, the pressure dependency of the melting temperature, and the heat of fusion (the enthalpy of fusion), the Clausius-Clapeyron equation was used to determine the volume changes on melting of the polymers at atmospheric pressure DeltaV(m0)(DTA). DeltaV(m0)(DTA) of NCH was lower than that of the nylon 6 alpha -form. The smaller DeltaV(m0)(DTA) of NCH was attributed to the presence of nylon 6 gamma -form in NCH. The values of DeltaV(m0)(DTA) for nylon 6 and poly(butylene terephthalate) were similar to those obtained from pressure-volume-temperature relationships DeltaV(m0)(PVT) of those polymers. The entropies of fusion were constant and independent of pressures up to 100 MPa. The volume changes on melting (DeltaV(m) in cm(3)/K) under high pressure can be approximately described by the following equation: DeltaV(m) = 0.165T(m)V(w) (dT(m)/dP)/ (dT(m0)/dP)/298, where T-m and T-m0 are the melting temperatures (in K) under high pressure and atmospheric pressure, respectively and V-w is the van der Waals volume of the polymer (cm(3)/g).