Reactions of 9-ethylguanine, 2′-deoxyguanosine and guanosine with bromomalondialdehyde in aqueous buffers over a wide pH-range were studied. The main products were isolated and characterized by ¹H and ¹³C NMR and mass spectroscopy. The final products formed under acidic and basic conditions were different, but they shared the common feature of being derived from glyoxal. Among the 1 : 1 adducts, 1,N²-(trans-1,2-dihydroxyethano)guanine adduct (6) predominated at pH < 6 and N²-carboxymethylguanine adduct (10a,b) at pH > 7. In addition to these, an N²-(4,5-dihydroxy-1,3-dioxolan-2-yl)methylene adduct (11a,b) and an N²-carboxymethyl-1,N²-(trans-1,2-dihydroxyethano)guanine adduct (12) were obtained at pH 10. The results of kinetic experiments suggest that bromomalondialdehyde is significantly decomposed to formic acid and glycolaldehyde under the conditions required to obtain guanine adducts. Glycolaldehyde is oxidized to glyoxal, which then modifies the guanine base more readily than bromomalondialdehyde. Besides the glyoxal-derived adducts, 1,N²-ethenoguanine (5a–c) and N²,3-ethenoguanine adducts (4a–c) were formed as minor products, and a transient accumulation of two unstable intermediates, tentatively identified as 1,N²-(1,2,2,3-tetrahydroxypropano) (8) and 1,N²-(2-formyl-1,2,3-trihydroxypropano) (9) adducts, was observed.