Hot and cold extraction methods for measuring pH values of paper have been in use for a number of years, but the available data do not define clearly the method that gives the best results for estimating the effects of acidity on a paper's rate of deterioration. Plotting heat-aged folding endurance data, for five or more test points, on semilogarithmic paper to form a regression line produces a more accurate method of determining a paper's rate of deterioration than the percentage retention based on the previously used two point (control and heated) method. When this new procedure is applied, the test data of heat-aged noncoated printing papers, writing grades, and card stock indicate that the cold extraction methods of making a pH gives the best results of the two methods for estimating a paper's rate of deterioration. -- AATA