The invention provides methods and kits for characterizing the activity of a methyltransferase or demethylase. The method involves enzymatically methylating or demethylating in vitro a substrate that is a peptide fragment of a full-length polypeptide, and then non-enzymatically methylating the peptide substrate with methyl groups that differ in molecular weight from the enzymatically added or removed methyl groups. Typically, deuterated or 13C formaldehyde is used to non-enzymatically methylate the substrate. The fully methylated substrate is then characterized by mass spectrometry to determine the ratio of enzymatically produced nonmethyl, monomethyl, and dimethyl residues on the peptide.