In the presence of formaldehyde and a mild reducing agent, reductive methylation is known to achieve near complete dimethylation of protein amino groups under non-denaturing conditions, in aqueous media. Amino methylation of proteins is employed in mass spectrometric, crystallographic, and NMR studies. Where biosynthetic labeling is prohibitive, amino (13)C-methylation provides an attractive option for monitoring folding, kinetics, protein-protein and protein-DNA interactions by NMR. Here, we demonstrate two improvements over traditional (13)C-reductive methylation schemes: (1) By judicious choice of stoichiometry and pH, ε-aminos can be preferentially monomethylated. Monomethyl tags are less perturbing and generally exhibit improved resolution over dimethyllysines, and (2) By use of deuterated reducing agents and (13)C-formaldehyde, amino groups can be labeled with (13)CH(2)D tags. Use of deutero-(13)C-formaldehyde affords either (13)CHD(2), or (13)CD(3) probes depending on choice of reducing agent. Making use of (13)C-(2)H scalar couplings, we demonstrate a filtering scheme that eliminates natural abundance (13)C signal.