Naphthalene spheres (particle diameter 2 < dn < 20 mm) were vaporized in beds fluidized by air at a temperature of about 65°C. The bed material consisted of inert glass beads or alumina in the size range 100 < dp < 700 μm. Mass transfer coefficients were measured by determining weight loss with time, as a function of dn, dp and the fluidization velocity U. The ratio dn/dp has been varied from 3 to 200. An interesting conclusion might be that there is no influence of the fluidization velocity on these transfer coefficients; they only depend on the minimum fluidization velocity Umf. The empirical correlation mf(jD)mfRemfm=0.105 + 1.505 (dn/dp)−1.05 with m = 0.35 + 0.29 (dn/dp)−0.50, as a best fit of all the results, is accurate within 15%. At dn/dp = 1 it links up very well with the results of Hsiung and Thodos. For large values of dn/dp agreement with known results of mass transfer measurements between a fluidized bed and a wall or an object is good.