Experiments were conducted to determine the values of Ez and Er which are dispersion coefficients of fluid in a gaseous fluidization and a liquid fluidization, respectively. Various kinds of solid particles, 1480mesh in size, (Cf. Table 1) were fluidized in three glass tubes having diameters of 6.7, 9.9 and 12.1cm, respectively. As a tracer fluid, ammonia or hydrochloric acid solution was used. The value of Ez was obtained by Gilliland et al's back-mixing method, and the value of Er by Hanratty et al's method. The results are given in Tables 2 and 3. Based on these experimental data, Eqs. (19) and (20) were derived, to express the values of Ez in gaseous fluidization, and Eq. (21), to express those of Er in gaseous fluidization. It was difficult to Ez in obtain good correlation in the case of the liquid fluidization, but approximate values of Ez and Er were derived from Eqs. (22) and (23), respectively.A theoretical equation, Eq. (2), was derived to represent the distribution of the concentrations of the tracer material introduced constantly from a point at the center of the bottom of the bed. Mixing efficiency, M, of fluid in the bed was defined by Eq. (3) on the basis of the dispersion of concentrations. Values of M, as calculated from Eq. (12), proved to be 5080% in the case of the gaseous fluidization and 1050% in that of the liquid fluidization, which served to make clear the extent of the mixing of fluid quantitatively.Experimental work was carried out in an attempt to evaluate the fraction void, e, in fluidized beds, and the expressions were obtained as in Eqs. (15)(17) for liquid fluidization, and as in Eq. (18) for gaseous fluidization.