A process for the synthesis of L-phenylalanines and analogues thereof from trans-cinnamate, ring substituted trans-cinnamates or various acrylate derivatives and ammonia or an ammonia donor in the presence of L-phenylalanine ammonia-lyase enzyme and a desensitizing agent such as polyhydric alcohols and polyethylene glycol-(400) is disclosed. The process embraces the discovery that compounds such as polyhydric alcohols and polyethylene glycol-(400) desensitize the L-phenyalanine ammonia-lyase enzyme to substantially higher substrate concentrations than practiced heretofore. In addition, polyhydric alcohols and polyethylene glycol-(400) substantially enhance the instantaneous rate of reaction and inhibit inactivation of the L-phenylalanine ammonia lyase enzyme over longer reaction periods.