The molar extinction coefficient at 210 nm (epsilon(210)) of the N-methylated analogs of A beta(30-40) was analyzed. Additive contributions from a 10-residue peptide main chain (epsilon(CONH)(210)) and modulation caused by N-methylation (Delta epsilon(210)) accounted for the observed results. The observed epsilon(CONH)(210) value was concordant with reported values. The difference in the Delta epsilon(210) value was explained in terms of changes in electronic structure and conformational restriction resulting from N-methylation.