AbstractSix polystyrene fractions of molecular weights ranging from 1,300 to 250,000 were dissolved in amyl acetate a t various temperatures. The velocity of dissolution was measured by the increase in the refractive index. An assumption was made to describe the experimental results, namely, that the adhesive layer on the surface of solid polystyrene in solvent consists of macromolecular coils the diameter of which can be calculated by Flory's formula. Comparison of the value of the exponentA(fromMA) obtained from viscosity measurements with the value calculated from the velocity of dissolution shows excellent agreement. The temperature dependence of the velocity of dissolution is dependent also on the diffusion process of the solvent molecules into the polymer. The expression for this diffusion derived by considering it, as usual, as a rate process, can be introduced into the equation for the velocity of dissolution. The resulting equation accurately describes the experimental results. The velocity of dissolution is different above and below the glass temperature of the polymer. It is possible to use the velocity of dissolution for a determination of molecular weight if the constant of the apparatus has been found by means of a standard sample of known molecular weight. This method helps in the difficult range of molecular weights from 5,000–40,000, where it is very sensitive, whereas other methods are no