Method of determination of penultimate model reactivity ratios has been generalized to take care of specific systems where copolymer composition data are available only in narrow ranges. A number of copolymerization systems reported in literature have been analyzed and applicability of generalized relationship was tested. Most systems are seen to follow penultimate model better than the terminal model. Monomer independent universal relationship of penultimate model reactivity ratios has similarly been generalized and successfully tested. (c) 2007 Elsevier Ltd. All rights reserved.