The goal of this study is to determine the effect of particle thermal time constants on fluidized bed convection coefficients and explain this behavior in terms of mechanistic models of particle convection. Heat transfer coefficients from a vertically oriented, cylindrical heat transfer probe were measured in beds of steel or bismuth particles fluidized in either nitrogen or carbon dioxide. Nusselt numbers were measured as functions of superficial velocities in the fluidized beds, corrected for voidage, consistent with either single-particle or packet theories of heat transfer in fluidized beds, and plotted against the dimensionless velocity ratio. The results of this study suggest that residence times of particles or packets do not increase regularly with increasing superficial velocity, but may increase rather abruptly above the minimum fluidization velocity and then become relatively constant.