Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume (DeltaV*) of a syndioregic main-chain hydrazone (SMCH) nonlinear optical polymer. The decay of the SHG signal from poled films of SMCH was recorded at hydrostatic pressures up to 2924 atm and at temperatures between 25 degreesC below the glass-transition temperature (T-g) to 20 degreesC above it. DeltaV* for pressures less than 500-1000 atm and T > T-g decreased as the temperature was increased. For pressures greater than 1000 atm, DeltaV* was essentially constant for all temperatures. In addition, the size of DeltaV* indicated that the chromophore in this main chain was internally flexible.