Methyl benzyloxyiminoacetate (MBOIA), a glyoxylic oxime ether, revealed different behaviors depending on the kinds of monomers used in the radical polymerization. MBOIA served as a retarder for styrene (St) and an inhibitor for vinyl acetate, whereas it showed little effect on the polymerization of methyl methacrylate. The retardation effect of MBOIA on the polymerization of St with dimethyl 2,2'-azobisisobutyrate (MAIB) was examined in detail in benzene. The rate constant (k(x)) of the reaction of MBOIA with polystyrene (PS) radical was 92 L/mol s at 50degreesC, 112 L/mol s at 60degreesC, and 143 L/mol s at 70degreesC, indicating that the reactivity of MBOIA toward PS radical is less than that of St by a factor of about 3. The Arrhenius plot of k(x) gave an activation energy of 20.3 kJ/mol. A nitrogen-centered radical of a stationary state was observed by electron spin resonance (ESR) in the polymerization of St with MAIB at 60degreesC in benzene in the presence of MBOIA, which is assignable to the radical (MBOIA .) formed by addition of PS radical to MBOIA. The stationary MBOIA . concentration increased with increasing MBOIA concentration and then tended to be saturated at high concentrations. The rate constant of termination between MBOIA . radicals was 1.87 x 10(5) L/mol s at 60degreesC with ESR.