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The methods generally used for determination of the molecular weight of polymers usually cannot be used for molecular weights between 1000 and 30,000. This is the range most suitable for determination by the isopiestic method, whereby the molecular weight is measured by comparing the vapor pressure of the polymer solution with that of a solution containing a reference solute. The isopiestic principle can be adapted to polymers if the high degree of dependence of solute and solvent activities upon concentration can be evaluated. This is easily accomplished by plotting the reciprocal of the apparent molecular weight against the concentration and making an extrapolation to infinite dilution. Certain modifications of the conventional isopiestic techniques are necessary in order to accelerate the attainment of isopiestic equilibrium and to eliminate water from the solutions. Isopiestic equilibrium is readily attained if a proper combination of solvent and temperature is chosen, and if the solutions are agitated adequately. Water may be removed by the use of a drying agent in the solutions, although the drying agent causes minor errors which are difficult to correct. Results obtained with the isopiestic method show that it is reliable for molecular weights up to 40,000. The accuracy decreases rapidly with higher molecular weights.