Free radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) in solution of benzene-d(6) using benzoyl peroxide (BPO) as the initiator was studied with on-line H-1 NMR kinetic experiments at 60 degrees C. It was observed that composition drifts in the comonomer mixture with reaction progress is significant. Hence, the monomer reactivity ratios of VA/MA system could be calculated by the data collected only from one sample via on-line following the comonomer mixture and copolymer compositions at different reaction time intervals up to medium overall monomer conversions. The results were in good agreement with the literature data reported for this system. The good fitting between theoretical and experimental changes in the comonomer mixture compositions as a function of reaction progress was observed, indicating the accuracy of the monomer reactivity ratios calculated by the new procedure presented here. (c) 2006 Elsevier Ltd. All rights reserved.