Carbon monoxide oxidation experiments were carried out over Pt-alumina catalyst particles of various sizes. The width of the conversion-temperature hysteresis loop goes through a maximum as the degree of intrapellet diffusion resistances (i.e., catalyst particle size) is varied. No hysteresis was observed over finely powdered catalysts. The above observations are in agreement with the qualitative predictions of diffusion-reaction theory, and provide further evidence to suggest that the steady-state multiplicities observed here were caused by the interactions between reaction and intrapellet diffusion resistances. The hysteresis loop was found to shift along the temperature axis as the pellet size was varied. The hysteresis occurred at the lowest temperature when catalyst particles of an intermediate size were used. Both larger and smaller particles showed multiplicity at higher temperatures. This observation is also consistent with diffusion-reaction theory. © 1979 Academic Press, Inc. All rights reserved.