The copolymerizations of allyl methacrylate (AMA) with methyl methacrylate (MMA) and of AMA with N-vinyl-2-pyrrolidone (VP) have been effected in bulk by gamma-irradiation to various stages of fractional conversion theta extending over a wide span of theta. Addition of n-heptane isolated the crosslinked copolymer. FT i.r. analysis on the supernatant solution yielded the composition of unreacted monomer mixture, which was used in conjunction with theta and the composition of initial feed to determine the reactivity ratios (r) via an iterative procedure involving a simplex algorithm, yielding AMA(1)/MMA(2) r(1) = 1.55 +/- 0.16, r(2) = 0.99 +/- 0.19 and AMA(1)/VP(2) r(1) = 1.408 +/- 0.074, r(2) = 0.401 +/- 0.044. For the system AMA/VP, analysis of crosslinked copolymer (via nitrogen analysis) was also possible and manipulation of the data by three modern procedures afforded good agreement with the values of r(1) and r(2) obtained by analysis of monomer mixture alone.