Tyrosine phenol‐lyase catalyzes the conversion of L‐tyrosine to phenol, pyruvate, and ammonia. The activation energy for the reaction was calculated to be 13,000 calories per mole. The heavy metal Cu++was found to result in a mixed type of inhibition (Ki 0.20 mM). The addition of mercaptoethanol was found to reverse the inhibition by Cu++. Competitive inhibition was found with the amino acids L‐alanine (Ki18 mM) and L‐phenylalanine (Ki 4.4 mM). Phenol, an end product of the tyrosine phenol‐lyase reaction, was also found to inhibit enzyme activity (Ki 50 mM). Tyrosine phenol lyase catalyzed the formation of pyruvate from L‐tyrosine methyl ester, S‐methyl‐L‐cysteine, and L‐serine but at rates lower than with L‐tyrosine. Km values for L‐tyrosine methyl ester, S‐methyl‐L‐cysteine, and L‐serine were found to be 0.37 mM, 0.40 mM, and 1.2 mM, respectively. The reverse reaction by which L‐tyrosine is produced from phenol, pyruvate, and ammonia was demonstrated. The pH optimum for the reverse reaction was found to be