The terminal and penultimate model reactivity ratios in the free-radical copolymerization of styrene and methyl methacrylate in bulk at 40 degrees C were calculated by means of the simplex and scanning methods. Calculations showed that for the terminal model r(1) and r(2) vary in comparatively narrow ranges of 0.548 - 0.552 and 0.480 - 0.483, respectively. For the penultimate model, the most accurate reactivity ratios calculated by the simplex method, which were r(11) = 0.727, r(22) = 0.490, r(21) = 2.890, r(12) = 4.583, are surrounded with sets of reactivity ratio values of equal accuracy. The ranges of variation were found to be 0.711 - 0.746, 0.487 - 0.492, 2.810 - 2.970 and 4.213 - 5.049, respectively. Numerical values of the penultimate r-parameters calculated with the simplex method depend, due to the structure of the multidimensional space (r(11), r(22), r(21), r(12), sigma), on the initial guess for the r-parameters. Use of the covariance matrix for the estimation of the indetermination ranges is discussed.