The monomer reactivity ratios for the copolymerizations of p-isopropylstyrene with styrene and with methyl methacrylate have been determined by the ionization chamber-vibrating reed electrometer radioactivity assay technique. The values from the differential form of the copolymerization equation are r1(styrene) = 1.22, r2(p-isopropylstyrene) = 0.89, and r1(methyl methacrylate) = 0.44, r2(p-isopropylstyrene) = 0.39. The values from the integrated form of the equation are r1(styrene) = 1.37 and r2= 0.99. These values indicate that, in the copolymerization of p-divinylbenzene (p-DVB) with styrene, the p-isopropylstyrene-like unit, formed from having the first vinyl group of p-DVB reacted, takes part in subsequent propagation reactions with styrene less readily than either styrene or p-DVB.