Monomer reactivity ratios in free-radical copolymerization of styrene and 4-vinylpyridine monomers at 80-degrees-C in toluene solution under reduced pressure were determined for low and high conversions. Finemann-Ross, Kelen-Tudos, extended Kelen-Tudos and Mayo-Lewis methods were used for this purpose. The most reliable method was found to be the extended Kelen-Tudos method. By the use of this method, for conversions below 20%, monomer reactivity values of 0.700 +/- 0.005 and 0.335 +/- 0.001 were calculated for 4-vinylpyridine and styrene respectively. For conversions greater than 20%, these values were found to reduce to 0.538 +/- 0.004 and 0.277 +/- 0.001 respectively.