To determine the reactivity ratios in the radical copolymerization of the reaction product of acetylsalicyclic acid with 2,3-epoxypropyl methacrylate and N-vinylpyrrolidone, all available methods are used and compared. Using as criteria the accuracy of the confidence interval and the impossibility of obtaining in any case negative values for a reactivity ratio, the Tidwell-Mortimer method is recommended.