In an attempt to clarify ambiguities in earlier reports on the preclinical chemistry of Antineoplaston A10 (3-[(phenylacetyl)amino]-2,6- piperidinedione; PAP), we detail herein hydrolysis studies with improved synthesis and characterization of PAP hydrolysis products, (phenylacetyl)- glutamine (PAG), (phenylacetyl)isoglutamine (PAIG), and (phenylacetyl)- glutamic acid (PAGA). Flash chromatography proved superior to extraction in the isolation of synthetic standards and hydrolysates. Synthesis of PAIG directly from commercial isoglutamine showed consistently better yields than the previously reported method. The 1H and 13C NMR and HPLC-MS data from the synthesized standards matched the data from the isolated PAP hydrolysates formed under acid and alkaline degradation conditions. Multiple quantum coherence NMR methods (HMQC and HMBC) and HPLC-MS-MS methods were applied to provide unambiguous structural assignments for key isomers PAG and PAIG. Previous PAP hydrolysis studies are shown to be reproducible, and the structures of hydrolysis products are elucidated in detail.