Immunoassay of methyl tert-butyl ether (MTBE) and related fuel oxygenates and their breakdown products is performed using a monoclonal antibody produced using a hapten with a CH.sub.3--O--C(CH.sub.3).sub.2--CH2-moiety, preferably CH.sub.3--O--C(CH.sub.3).sub.2--(CH.sub.2).sub.3--CH(CH.sub.3)--CH.sub.2-- -CHO conjugated to a carrier protein.