The influence of temperature on the basic variables that define the fluidization process of a particle bed is analysed through experiments on several types of solids from ambient temperature to 800 degrees C. By means of independent measurements of voidage both at rest and at incipient fluidization, it is shown how the increase of the fixed bed voidage with T, in the total absence of gas flow, underlies the corresponding increase of voidage at incipient fluidization, causing a change in minimum fluidization velocity. Such an effect appears clearly related to a variation of interparticle forces with temperature, parallel to the thermal variation of gas properties. The paper demonstrates that the predict:ive ability of some classical equations of fluidization theory, such as Carman-Kozeny's and Ergun's, is fully restored if, in addition to the change of gas density and viscosity, the dependence of epsilon(mf) on temperature is correctly accounted for.