An in vitro dissolution procedure for phenytoin (5,5-diphenylhydantoin) products, which utilizes a UV method of analysis, is compared with an HPLC method. The UV method is subject to interference due to inactive materials and decomposition products. One of the inactive materials was identified as the synthetic precursor, benzophenone. The decomposed and degraded capsule contained brown material probably due to alkaline hydrolysis of lactose in the capsule. Both benzophenone and the brown material had significant UV absorption in the phenytoin region. Consequently, a simple, specific, and sensitive HPLC method for dissolution testing of phenytoin has been developed. Comparison of the results of the UV and HPLC methods indicates that the UV method may result in up to 51% higher dissolution values, depending on the presence of inactive ingredients and the purity of the product.