The origin of the instability observed in the bromate-sulfite-proton system is discussed. This is known as the subsystem of some pH oscillators which provides the positive feedback channel for H+. It is composed of the following two fast equilibria : H+ + SO32- <-> HSO3- and H+ + HSO3- <-> H2SO3 and the oxidation of HSO3- and H2SO3 by BrO3- to recover the initially supplied proton as 3HSO(3-) + BrO3- --> 3SO(4)(2-) + Br- + 3H(+) and 3H(2)SO(3) + BrO3- --> 3SO(4)(2-) + Br- + 6H(+). This scheme exhibits bistability under flow conditions. It gives rise to chemical oscillations if it is coupled with a simple linear decay process of H+ as a negative feedback channel. It is also capable of exhibiting chaotic behavior in the presence of HCO3-. We analyze the nature of the strong nonlinearity provided by the above scheme in detail and show that it arises essentially from the fast reaction of H2SO3 with bromate. The simplicity of the scheme enables us to describe the system dynamics with three variables without any approximation. As a result, we now have a model in hand in which we can compare the theoretical results directly and quantitatively with the corresponding experiments. This situation may open a way to understand the complexity in nonlinear chemical systems in a more quantitative manner. Some recent experimental findings of complex oscillatory behavior including chaos for the system with more realistic negative feedback channels, or for the system in which BrO3- is replaced with H2O2, are discussed on the basis of the present analysis.