A physicochemical procedure for the analysis of vidarabine in aqueous parenteral formulations was needed to assure potency and to define stability. Concurrent with the development of this method, its decomposition products and route in aqueous solution were determined. A quantitative procedure was developed to determine intact drug in the presence of decomposition products, and the results obtained were validated by microbial assay. Spectral (UV and polarimetric) and TLC evidence indicated that, in aqueous solution, hydrolysis without racemization occurs, yielding adenine and arabinose. The sensitivity of the method to decomposition is improved by ion-exchange separation of adenine and drug before UV measurement. Analysis of partially decomposed solutions of the drug by both ion-exchange and microbiological methods gave comparable results.