4-Vinylbenzene sulfonic acid sodium salt (VB) - Acrylamide (Aam) homopolymerization and copolymerization reactions with various feed ratios were performed in water at 60 degrees C. All reactions were monitored online, by an Automatic Continuous Online Monitoring of Polymerization (ACOMP) system. The concentrations of the two comonomers in their monomeric form, as well as their concentrations incorporated into polymer, were obtained from the monitored data. It was seen that Aam homopolymerization was faster than VB and both homopolymerization rates were higher than copolymerization rates at any combination. in the reactions with 1.5%, 5% and 10% (mol) VB, VB was completely depleted and further reaction was Aam homopolymerization. in the copolymerizations with 5 to 50% initial VB content, the plots of VB fraction in the remaining monomer mixture versus conversion exhibited a corner at 10-30% conversion. This corner showed that the behavior of the reaction changed abruptly at this point. in the first stage of the reactions, the composition was seen to be almost constant with almost no composition drift whereas in the second part, the VB fraction decreased monotonically. Monomer reactivity ratios (MRR) were calculated by error in variables (EVM) method. Since the reactions gave two distinguishable regions, the reaction part before and after the corner were evaluated separately. Therefore, the monomer reactivity ratios were found as r(Aam) = 0.34 +/- 0.07, r(VB) = 0.40 +/- 0.21 for the part before and r(Aam) = 0.2 +/- 0.04, r(VB) = 9.0 +/- 0.8 for the part after the corner.