In the presence of the metalloradical (C5Ph5)Cr(CO)(3)(center dot) thermolysis of the radical source AIBMe (dimethyl 2,2'-azobis(isobutyrate)) gave MMA and (C5Ph5)Cr(CO)(3)H. A high concentration of (C5Ph5)Cr(CO)(3)(center dot) discouraged chain growth when AIBMe was heated in MMA, making the monomeric MMA radical (i.e., methylisobutyryl) the dominant organic radical and enabling us to measure the H-center dot transfer rate constant, k(tr)(monomer), from the methylisobutyryl radical to (C5Ph5)Cr(CO)(3)(center dot). The result, 5.2 x 10(5) M-1 s(-1) at 70 degrees C, is smaller than the chain transfer rate constant previously determined for chain-carrying radicals during MMA polymerizations (1.4 x 10(6) M-1 s(-1) at 70 degrees C).