An expression for the rise velocity of single circular-cap gas bubbles in two-dimensional (2D) beds consisting of powders or liquids is developed with the aid of experimental data and computational fluid dynamics. Experiments were performed in a two-dimensional rectangular column of width D-T = 0.3 m by injecting air bubbles in fluidised beds of silica (mean particle size, d(p),= 38 mu m) and polystyrene (mean particle size, d(p) = 570 mu m) and in water. The rise velocity of single gas bubbles in the size range d(b) = 0.015-0.12 m were found to decrease significantly with increasing ratio of bubble diameter to bed width, d(b)/D-T. Computational fluid dynamics simulations of single gas bubbles rising in water, carried out using the volume-of-fluid (VOF) method, showed good agreement with experiment and were used to develop a common expression for the rise velocity of single gas bubbles in gas-solid fluidised beds and bubble columns. The 2D circular-cap bubble rise velocity is found to similar to 10-30% lower than that of a 3D spherical-cap bubble having the same equivalent diameter.