Existing models of the ferrocyanide-iodate-sulfite (FIS) reaction seek to replicate the oscillatory pH behavior that occurs in open systems. These models exhibit significant differences in the amplitudes and waveforms of the concentration oscillations of such intermediates as I-, I-3(-), and Fe(CN)(6)(3-) under identical conditions and do not include several experimentally found intermediates. Here we report measurements of sulfite concentrations during an oscillatory cycle. Knowing the correct concentration of sulfite over the course of a period is important because sulfite is the main component that determines the buffer capacity, the pH extrema, and the amount of oxidizer (iodate) required for the transition to low pH. On the basis of this new result and recent experimental findings on the rate laws and intermediates of component processes, taken from the literature, we propose a mass action kinetics model that attempts to faithfully represent the,Chemistry of the FIS reaction. This new comprehensive mechanism reproduces the pH oscillations and the periodic behavior in [Fe(CN)(6)(3-)], [I-3(-)], [I-], and [SO32-](T) with characteristics similar to those seen in experiments in both CSTR and semibatch arrangements. The parameter ranges at which stationary and oscillatory behavior is exhibited also show good agreement with those of the experiments.