The radial distribution of bubbles in air-water bubbly flow was experimentally investigated using a dual optical probe and hot-film anemometry. The experiments were performed in a 200mm diameter vertical pipe at liquid superficial velocity in the range of 0.2 to 0.68m/s and gas superficial velocity from 0.005 to 0.18m/s. At low void fraction flows, the radial void fraction profiles show a wall peak. As the void fraction increases the profiles tend to peak at the pipe centreline. The distribution of the net radial interfacial forces for both core and wall peak void distributions was obtained using the two-fluid model. The radial direction of the bubble migration in the pipe core was related to the ratio between the turbulent dispersion and the lift forces. The effect of the pipe diameter on this ratio was investigated.