The complexation of bovine serum albumin (BSA) with poly(N-isopropylacrylamide) (PNIPAM) in an aqueous system (pH 7) has been investigated by means of fluorescence spectroscopy. Through analyzing the change of fluorescence intensity of protein chromophores caused by complexation, a quantitative mathematical equation has been established and the average number of bound proteins per neutral polymer (n(b)) can be calculated accurately without destroying the dynamic equilibrium of aqueous complex system. At the same time, with the help of this equation, the extreme value of nb can also be calculated when PNIPAM concentration is low enough. Compared with traditional calculation methods, this method has the advantages of rapid detectability, high sensitivity, accuracy and extensive applicability. Thus, it is a better way to study the complexation of protein with polymer. (c) 2008 Elsevier Ltd. All rights reserved.