An integrodifferential estimation approach is applied to draw the reactivity ratio (RR) dependencies on the monomer feed composition. The approach combines the integral method robustness with the differential method discrimination capability and includes an uncertainty assessment in terms of the number, location, and quality of experimental data. The proposed technique is applied to three copolymerization systems: one, butyl acrylate/methyl methacrylate, with composition measurements, and two, p-chlorostyrene/styrene (PCS/STY) and PCS/p-methoxystyrene, with composition and average propagation measurements. In most cases, the RR pair exhibited a dependency on composition, with a behavior that is different from the one predicted by the penultimate model. In one case (PCS/STY), the behavior resembled the one of the bootstrap model. The results are put into perspective with the ones reached previously with the integral method.