With a Monte Carlo simulation method, copolymer properties have been thoroughly studied, and the influence of the reactivity ratios and feed composition has been taken into consideration. Instantaneous alterations of the copolymer composition and copolymer heterogeneity, which is also called a randomness parameter, have been examined with data obtained from the simulation at each stage of the copolymerization reaction. The results prove the azeotropic behavior of copolymerization reactions in which both reactivity ratios are greater than unity, although some special reactivity ratio combinations ignore the azeotropic behavior. The copolymer composition reaches an azeotrope point at the end of the copolymerization reaction when the copolymerization is an azeotropic reaction. In addition, the randomness parameter takes its maximum value at the azeotrope point when reactivity ratio r(A) is equal to reactivity ratio r(B). Finally, increasing the reactivity ratios causes no change in the trend of copolymer composition/feed composition curves when r(A) is equal to r(B). However, the curves produced with larger r(A) and r(B) values show more fluctuations. (C) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 106: 4138-4147, 2007