The substrate specificity of catechol oxidase from Lycopus europaeus towards phenols is examined. The enzyme catalyzes the oxidation of o-diphenols to o-quinones without hydroxylating monophenols, the additional activity of tyrosinase. Substrates containing a -COOH group are inhibitors for catechol oxidase. The products of enzymic oxidation of caffeic acid were analyzed and isolated by HPLC with diode array detection. The neolignans of the 2,3-dihydro-1,4-benzodioxin type (3, 6-8), 6,7-dihydroxy-1-(3,4-dihydroxyphenyl)-2,3-dicarboxy-1,2-dihydro naphthaline (1) 6,7-dihydroxy-1-(3,4-dihydroxyphenyl)-3-carboxynaphthaline (5) and 2,6-bis-(3',4'-dihydroxyphenyl)-1-carboxy-3-oxacyclo-(3,0)-pent an-2-on-1-ene (4) were formed. A reaction mechanism for the formation of (1, 4 and 5) is discussed.