We investigated the effect of bubble size on the drift-flux parameters at low liquid flow conditions by measuring the radial profiles of void fraction and phase velocities in a vertical bubbly pipe flow of diameter D-p = 72 mm and height H = 18 m. To study the effect of the bubble size we used two different types of bubble inlets. We measured the local bubble fraction and velocity U, by using single and four-point-optical fibre probes, and we used Laser Doppler Anemometry to determine the liquid velocity U-l. The distribution parameter C-0 and the weighted mean drift velocity \U-drift\ were directly computed from the local measurements at a height h = 5 m on our experimental set-up. Both parameters were influenced by the bubble size. Provided no liquid flow reversal occurred at the near wall region, the distribution parameter reached a below unity minimum plateau value of C-0 = 0.95 for wall peaking void fraction profiles. At low liquid input conditions both the liquid input and bubble size had an influence on the distribution parameter. Extreme values such as C-0 > 2 were measured. From these measurements we developed models for the drift-flux parameters to take into account the effect of bubble size and input-flow conditions for our intermediate pipe diameter value. These models were tested and validated with separately collected experimental data. (C) 2004 Elsevier Ltd. All rights reserved.