Previously published material on the alpha-methyl styrene/methyl methacrylate (alpha-MS/MMA) copolymer system at temperatures above the ceiling temperature of alpha-MS has focused on low-conversion results. Several attempts have been made to estimate copolymer reactivity ratios from experimental data, but in most cases errors are present in the determination of copolymer composition variables. In this article, the results of rigorous parameter estimations, as applied to two sets of equations developed independently by P. Wittmer (Adv Chem 1971, 99, 140-174) and H. Kruger, J. Bauer, and J. Rubner (Makromol Chem 1987, 188, 2163-2175), are discussed. Experimental data for the copolymer system at low conversions, as well as over the full conversion range, are presented, covering a temperature range of 60-140 degrees C. A comparison of the data trends with traditional copolymer systems indicates that the reversibility of both MMA and alpha-MS must be considered when composition, polymerization rate, or molecular weight equations are being developed.