The rate of the water-catalyzed hydrolysis of three activated amides in aqueous solution is significantly retarded by small amounts of a-phenylalanine as compared with the rate acceleration induced by other common a-amino acids not containing a benzyl-group in their side chain. These contrasting effects emphasize the large hydrophobicity of a-phenylalanine and are of relevance for a better quantitative understanding of protein folding and molecular recognition processes involving proteins.