The growing interest in acid deposition has led to a proliferation of laboratories engaged in such studies. High-level quality assurance (QA) procedures are required for each program to standardize the diverse measurement methods in use and to determine the validity of differences in measurements widely separated in space and time. Both inlaboratory (quality control) and external (quality assurance) procedures are required. A complete QA program for acid precipitation measurements must address program objectives; site selection and operation; operator selection and training; sample collection, handling, and analyses; and data checking, storage, retrieval, and transmission. Objective criteria must be developed for detecting adulterated samples and invalid data. Appropriate laboratory and field blanks must be collected and analyzed. Standard techniques (sample spiking, replicate analysis of standards and samples) should ensure the reliability of analytical results. Relevant quality assurance data, including analytical detection limits, blank values, and the variability of replicate determinations, must be supplied with each data transmittal. Experimental information should be available upon request. The measurement of the pH of dilute solutions such as rain is particularly difficult; differences as large as 0.3 pH unit may be observed in replicate analyses of the same sample using different electrode types. Laboratory results are presented demonstrating typical variability to be expected in the collection, storage, and analysis of rainwater for major ions, including hydrogen ion.