A new error-in-variables method was developed to estimate the reactivity ratios in copolymerization systems. It brings the power of automatic, continuous, on-line monitoring of polymerization (ACOMP) to copolymerization calculations. In ACOMP systems, monomer and polymer concentrations are measured by the monitoring of two independent properties of the system. The reactivity ratios are found by taking into account errors in the monomer concentrations determined from measurements and from calibration of the instruments. All the error sources are taken into account according to the error-in-variables method, and their effects are reflected in determining the confidence intervals of the reactivity ratios by the usual error propagation technique.