To better understand the nitromethane decomposition mechanism over Cu-ZSM-5 and Co-ZSM-5, CH3NO2 and CO adsorption on Cu-ZSM-5 and Co-ZSM-5 was studied by IR and EPR spectroscopies. After adsorption of nitromethane on H-ZSM-5, Al(III)–NCO forms at 200°C. Nitromethane adsorption on Cu-ZSM-5 forms Si–NCO and Cu(I)–NCO which are observed from room temperature to 300°C. Nitromethane adsorption on Co-ZSM-5 forms Si–NCO from room temperature to 200°C, and Co(II)–NCO above 200°C. We suggest that Co(II)–NCO forms by the substitution of NCO for water in dehydrated Co-ZSM-5 from 200 to 450°C. CO is adsorbed on Cu-ZSM-5 to form Cu(I)–CO (2157 cm−1) at room temperature. It is newly found that CO binds to Cu(II)–O− sites in dehydrated Cu-ZSM-5 at room temperature to form Cu(II)–O−(H2O)(CO) (2138 cm−1). But only Cu(I)–CO (2157 cm−1) remains at 200°C and above. CO adsorption on Co-ZSM-5 does not bind to Co(II) and only shows a gas phase CO IR spectrum. CO adsorbed on dehydrated Co-ZSM-5 at − 196°C forms weakly bound Co(II)–CO by EPR, in which the unpaired electron is mainly located on Co(II). Desorption of CO from Co(II)–CO occurs by about − 73°C so Co(II)–CO is not observed at room temperature.