Abstract Pressure drop has been determined experimentally for downward co-current annular two-phase flow, with water and air flow Reynolds numbers in the ranges 5100–27 200 and 3400–21 600, respectively. In order to investigate the effect of tube diameter, four tubes, of I.D.'s ranging from 1.56–4.12 cm, were tested. Special care was taken to eliminate effects of surfactants. The resulting skin friction coefficients were only in qualitative agreement with most previously reported data. At a fixed gas Reynolds number, a large increase in friction accompanied a decrease in tube diameter. Existing correlations were found to be unsuccessful for the present data.