The optically detected magnetic resonance (ODMR) spectra of the covalent DNA adduct of 9,10-epoxy-9,10,11,12-tetrahydrobenzo[e]pyrene (BePE) reveal that the excited triplet state chromophore is perturbed by the nucleic acid and that this perturbation is diminished successively by denaturation and enzymatic hydrolysis of the modified DNA, indicating that the adduct resides in an environment with some quasi-intercalative character. In contrast the covalent adduct of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene shows little ODMR evidence of interaction with the nucleic acid, which, in view of ODMR's sensitivity demonstrated for the BePE adduct, suggests that the chromophore is situated in an environment resembling the bulk solvent. These results demonstrate that ODMR is more sensitive than conventional phosphorescence techniques to interactions between these pyrene-like chromophores and DNA.