Knowledge of bed expansions is important in the design and operation of gas-solid fluidized beds. This paper presents a study on the estimation of expanded bed height in a large three-dimensional gas-fluidized bed with a square section of 0.61 x 0.61 m(2). All experiments were performed at the freely bubbling mode and the bed expansions were recorded by a video camera. Bed materials were used 593 mu m raw perlite and 1233 mu m sand falling within the categories of Geldart's Groups B and D, respectively. The bed height at minimum fluidization ranged from 0.0398 to 0.3176 m, while the excess air velocity from 0.034 m s(-1) to 0.7453 m s(-1). Equations related to the bed expansion were given using a modified form of two-phase theory of fluidization. A correlation for the average bed expansion (void fraction) was also presented that has been derived from the principal form found successful in gas-liquid systems as follows: R = 0.5482 d(p)(-0.129) (U-o - U-mf)(0.111) with an average deviation of less than 1%. The experimental findings were compared with previously reported results and were discussed in the light of available correlations.