S-adenosyl-L-methionine-dependent methyltransferase that preferentially catalyzes the methylation of 3-OH phenolic compounds like isovanillic acid and 3-OH-4-Met cinnamic acid. May play a role in promoting lignin degradation by methylating and inactivating free-hydroxyl phenolic compounds, products of lignin cleavage which are known inhibitors of lignin peroxidases. Optimum pH is 8.0. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.