The copolymerization reactivity ratios of acrylic acid and acrylamide are found at pH 5 and pH 2. Automatic continuous online monitoring of polymerization reactions (ACOMP) has been used for the first time to monitor the synthesis of polyelectrolytic copolymers. The composition drift during the reactions revealed that at pH 5, the acrylamide participates more in the copolymer, and at pH 2, the acrylic acid incorporates in the system at a higher ratio. The copolymerization data were analyzed by a recent error in variables (EVM) type calculation method developed for obtaining the reactivity ratios by on-line monitoring and gave at pH 5 reactivity ratios r(Aam) = 1.88 +/- 0.17, r(Aac) = 0.80 +/- 0.07 and at pH 2 r(Aam) = 0.16 +/- 0.04, r(Aac) = 0.88 +/- 0.08. The results show that the reactivity ratios depend strongly on the pH of the medium. The effect of polyelectrolytic interactions on the reactivity ratios is discussed in detail. (c) 2006 Wiley Periodicals, Inc.