The effect of a hydrogen-donor solvent (tetralin) on the thermal degradation of poly(styrene-allyl alcohol) in solution was investigated in a steady-state tubular flow reactor at 1000 psig (6.8 MPa), at various tetralin concentrations (0–50%), polymer concentrations (1–4 g/L), and temperatures (130–200 °C). The molecular weight distributions of the effluent at each condition were examined as a function of residence time by gel permeation chromatography. In the presence of tetralin, the polymer degrades by depolymerization to specific low molecular weight compounds and by random chain scission. No reaction was observed in the solvent 1-butanol in the absence of tetralin. The experimental data were interpreted with a rate expression first-order in polymer concentration based on continuous mixture kinetics, and rate coefficients were determined for the specific and random degradation processes. Activation energies were in the range of 5–10 kcal/mol for specific degradation and 33 kcal/mol for the random degradation process. A plot of rate coefficients versus tetralin concentration indicates a first-order rate at low tetralin concentrations and a zero-order dependence at high tetralin concentrations. © 1995, American Chemical Society. All rights reserved.