Part A of this thesis describes an experimental study of the initial motion of a bubble in a bubble dimensional air-solid fluidised bed. A technique is developed for releasing a cylindrical bubble (about 2" diameter), initially at rest, into an incipiently fluidised bed. The subsequent shape and motion of the bubble was followed by high-speed photography. This study was carried out in beds of glass beads of three different sizes with different supplementary air flow to the beds.An analysis of variance shows that the initial bubble acceleration was not affected by either particle size or supplementary air flow rates over the ranges investigated. The results suggest that starting from the cylindrical shape, a bubble reaches its final characteristic shape within a period of about 0.067 seconds (i.e. T = 1.3). Observed bubble profiles during transition are in good agreement with the theoretical shapes proposed by Murray for bubbles in fluidised beds and by Walters and DaviJson for bubbles in an inviscid liquid. The average initial acceleration of the centroid of the bubble is observed to be 0.77g compared with the average value of 0.67g observed by Walters and Davidson for "two dimensional" air bubbles in water, and a value of 1.0 g predicted by their theory (their Equation 14).Part B of this thesis is a theoretical investigation into the effect of voidage variation on the bubble throughflow velocity in a bubble in a fluidised bed. By extending Jackson's analysis of fluidisation, equations describing the bubble throughflow velocities are derived without the restrictive assumption of constant voidage outside the bubble. The mean throughflow velocities calculated from the present analysis are compared with those calculated from the constant voidage theories of Davidson and Murray. The limitations of the previous analysis (Leung and Sandford, 1969)(26) are also discussed.