We developed a new methodology to estimate the cross propagation rate constants and their confidence regions for the copolymerization of ethylene and 1-olefins using single site catalysts. We applied the method to a series of ethylene/1-octene copolymers. made with rac-Et(Ind)(2)ZrCl2/MAO in a solution reactor operated in semibatch mode. The method estimates the reactivity ratios using the Mayo Lewis equation and the cross-propagation rate constants with combination of the Mayo Lewis equation and a polymerization kinetics model based on monomer uptake curves. More importantly, our strict statistical treatment of the data allows us to estimate the joint confidence regions for these parameters, which is essential to establish the reliability of these estimates.