Free-radical copolymerization of vinyl pyrrolidone and acrylamide was performed in water at 60 degrees C. Their copolymerization reactions were continued up to high conversions and reaction mixture sampled sequentially during the reaction. The compositions of the aliquots were measured by UV spectroscopy, and their polymer contents were obtained by gravimetry. Data are evaluated by a recent error in variables method developed for sequential sampling. Reactivity ratios are found as r(A) = 2.03 +/- 0.14, r(VP) = 0.09 +/- 0.02. Analysis of the data up to 45% conversion by the Extended Kelen Tudos method gave consistent results. (c) 2006 Wiley Periodicals, Inc. J AppI Polyrn Sci 100: 3822-3827, 2006.