Abstract The mutagenicity of 1,2,5,6-diepoxycyclooctane (DECO) and 1,2,7,8-diepoxyoctane (DEO) was investigated using diploid Chinese hamster lung cells. 6-thioguanine (6-TG) resistance was used as the marker for mutagenicity testing: DEO was found to be genetically active; DECO, on the contrary, totally inactive. DEO readily formed adducts with radiolabeled nucleotides, while DECO failed to do so, as demonstrated through thin-layer chromatography (TLC) and the shift of the ultraviolet absorption maximum in DEO/nucleotide mixtures. The difference between the two compounds in chemical and genetic activities was attributed to their molecular conformations and the resulting differential flexibilities and adduct-forming abilities. Association between mutagenicity and adduct formation was conclusive.