Methyl methacrylate (MMA) has been copolymerized with ethylene dimethacrylate (EDMA) in bulk by gamma-irradiation to various stages of fractional conversion C-w (C-w = 0.001-0.901). On addition of n-heptane the cross-linked copolymer was isolated and FTIR measurements on the resultant solutions of unreacted monomer mixture in n-heptane afforded the composition (w(1)) of unreacted monomer mixture. Full details are reported of essential precautions and the appropriate calibration method. Although subsequent implementation of the data to calculate copolymer composition and thereby reactivity ratios is satisfactory in principle, this route was demonstrated to be inherently inaccurate in practice, especially at low values of C-w. Instead, C-w, w(1), and the initial monomer feed composition were employed to calculate the reactivity ratios by a specially devised nonlinear least squares program using a simplex algorithm, yielding r(MMA) = 0.674 +/- 0.045 and r(EDMA) = 1.34 +/- 0.18.