Two types of steady-state multiplicity patterns were observed during the isothermal oxidation of propylene over platinum wires. These patterns differ in the nature of the Ignition or extinction induced by slowly changing the propylene concentration. Changes in either the inert diluent, the wire temperature, or oxygen concentration caused a shift from one multiplicity pattern to the other. This information can be used to discard classes of kinetic models which cannot admit such a transition and to suggest the functional form of an appropriate rate expression. The process of model discrimination is greatly facilitated by the observation that the simplest model predicting this transition must have a pitchfork singularity. This information is utilized to develop the simplest rate expression which predicts the surprising shift in the multiplicity pattern.