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In many laboratory catalytic reactors light-o¤ curves describing the exit conversion versus inlet gas temperature do not follow the same path during ramp up and ramp down and this leads to dynamic hysteresis. This dynamic hysteresis, which may occur even under ideal conditions (e.g. plug ‡ow and negligible heat e¤ects) is fundamentally di¤erent from the steady-state hysteresis which is due to heat e¤ects and thermal feedback. We present expressions for the width of the dynamic hysteresis loop as a function of the ramp rate, solid to gas heat capacity ratio, space time and the heat Peclet number, for the limiting cases of pseudo-homogeneous and two phase plug ‡ow models with negligible heat e¤ects (T ad = 0). We review the conditions under which steady-state hysteresis can exist for a …nite T ad and demonstrate the combined impact of steady-state and dynamic hysteresis e¤ects. We study the light-o¤ behavior and dynamic hysteresis during the co-oxidation of CO + C 2 H 6 mixture on P t=Al 2 O 3. It is found that 1% of CO alone ignites at 473K whereas 500 ppm of C 2 H 6 alone ignites at 723K. The exotherm generated by the CO oxidation can reduce the ignition temperature of C 2 H 6 during the co-oxidation of CO and C 2 H 6. Bifurcation diagrams of solid exit temperature versus ‡uid inlet temperature for di¤erent inlet compositions illustrate the simultaneous as well as separate ignitions for CO and C 2 H 6. Each 1% increase in the CO feed mole percentage decreases C 2 H 6

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