The composition of novel terpolymers containing acrylamide (A), acryloyloxyethyltrimethylammonium chloride (Q), and bis-1,3(N,N,N-trimethylammonium)-2-propylmethacrylate dichloride (M) was analyzed combining FTIR and potentiometric titration. Calibration with A–Q and A–M copolymers of known composition was the prerequisite for the quantitative analysis of the terpolymers by FTIR. The analysis revealed deviations from the initial monomer feed composition resulting from different monomer reactivity. The differences became more pronounced when M increased in the feed. This suggests lowest reactivity for M. Lower intrinsic viscosity [η] for higher fraction of M in the feed confirmed the suggestion. The low reactivity results from the electrostatic influence of the two charges of M on the chain propagation, yielding shorter chains for more double-charged M in the monomer feed. For all terpolymers, the counterion activity was lower than theoretically expected and did not correlate with the average charge density. Heterogeneous charge distribution was hypothesized as the reason.