In this study, the bed expansion of an anaerobic fluidised-bed bioreactor was investigated. The experiments were conducted in a 4.4 cm diameter column with 0.152 mm sand as medium support of the biofilm. The temperature was maintained at 30°C and synthetic wastewater was used. The bed expansion data from the present study and literature data were used to evaluate the applicability of the correlations in predicting the bed expansion of the bioreactors. A correlation between bed expansion or voidage (ɛ) and the particle Reynold's (Re) and Gallileo's (Ga) numbers proposed by Setiadi (1989) ɛ = 1.72 Re0.203 Ga−0.179 was found to predict the bed expansion of anaerobic fluidised-bed bioreactors with better accuracy than any previously published correlations.