A standard titrimetric method used to determine acidity of surface and waste waters involves acidification of a sample (if necessary), followed by treatment of that sample with 30% hydrogen peroxide while heating, and finally, titration of the treated sample with base to pH 8.2. Such a method measures mineral acidity plus additional acidity due to the oxidation and hydrolysis of certain metal cations. While this type of method is commonly used to determine the acidity of mine drainage, the complex matrix presented by mine drainage often results in the formation of precipitates, a recognized interference in the method. In addition, the use of hot 30% hydrogen peroxide is not representative of redox conditions to which mine drainage is exposed in the field. While careful application of the standard titrimetric method provides acceptable precision in replicate acidity measurements, a systematic theoretical and experimental approach has been used to evaluate the impacts of the composition of the matrix, the extent of oxidation during sample preparation, and the formation of precipitates during titration on the accuracy of the method.