Particle dispersion and fluctuations were investigated in a liquidsolid fluidised bed with viscous liquid media by employing the relaxation method based on the stochastic model. The bed expansion or contraction during the transient state was analysed by means of the histogram of pressure drop variation with elapsed time. The relaxation behaviour of the liquidsolid fluidised bed appeared to be heterogeneous followed by homogeneous expansion or contraction. Effects of liquid velocity (UL), particle size (dP), liquid viscosity (mu L) and liquid holdup or bed porosity (epsilon L) on the fluctuating frequency (F) and dispersion coefficient (DP) of fluidised solid particles were examined. The values of F and DP increased with an increase in dP but decreased with mu L and exhibited local maxima with variations of UL and epsilon L. The flow pattern of solid particles could be changed from uniform or pseudo-homogeneous to turbulent random behaviour with an increase in the liquid velocity and liquid holdup or bed porosity. (c) 2012 Canadian Society for Chemical Engineering