Refractive indexes are used to allow the in-line monitoring of acrylic acid (AA)/vinyl acetate (VA) copolymerizations in azeotropic solutions of ethanol and water (ETOH). It is shown that refractive indexes of AA and VA solutions in polar and non-polar solvents vary almost linearly with monomer volume fraction and that correlations for polymer refractive index increments obtained by measuring the refractive indexes of polymer solutions in different solvents do not allow precise evaluation of monomer conversion in homopolymerization reactions. A procedure which consists of obtaining the refractive index parameters in homopolymerization reactions and extending the results to copolymerizations is successfully implemented for AA/VA copolymerizations in ETOH, allowing the simultaneous in-line evaluation of both monomer conversion and copolymer composition. (C) 1997 Elsevier Science Ltd.