Problems in the current experimental determination of radical reactivity ratios (i.e., 's values') in free-radical copolymerization and the resulting limited physical meaning of these reactivity ratios are highlighted in the present work. It is exemplified that the radical reactivity ratios obtained via fitting of average propagation rate coefficients are very sensitive to small uncertainties in monomer reactivity ratios and homopropagation rate coefficients. Furthermore, the failure to select an appropriate modelling method, and to design experiments so as to satisfy its assumptions, may lead to incorrect parameter estimates. These problems result in large uncertainties in the measured values of the radical reactivity ratios, which in turn lead to large uncertainties in calculated ratios of propagating radical concentrations. This suggests that experimental determinations of the relative radical concentrations may lead to complimentary means for obtaining information on radical reactivity ratios, andd possible ways of obtaining this information through catalytic chain transfer experiments are discussed.