In copolymerization systems with implicit penultimate effect, there are two radical reactivity ratios, s(a) and s(b), which influence the reaction ratios, r(a) and r(b), which govern the copolymer composition. Here, an error in variables method has been developed to determine s(a) and s(b). It is based on continuous on-line monitoring of two independent properties of the system. The ratios and the corresponding chi(2) values were found by taking into account errors emanating from measurements and from calibration from the instruments. It is shown that the kinetic data allows both ratios to be found if both monomer reactivity ratios are less than one. If the system is near ideality (r(a)r(b)similar or equal to 1) or if both reactivities are greater than one, only an average radical reactivity ratio, (s) over bar, can be reliably determined.