An experimental study on void fraction in a small diameter tube (9.525 mm) for vertical upward and downward, gas-liquid, two-phase flow was carried out. Void fraction measurements in water-air were taken for both upward and downward flow using a single beam gamma densitometer. The gamma densitometer was calibrated against the reading from a quick-closing valve system. The experimental results indicate that for V(SL) greater-than-or-equal-to a 1.7 m/s, the void fraction in both upward and downward flow is almost independent of the liquid flow rate. For downward flow and V(SL) less-than-or-equal-to 0.1 m/s, the void fraction is independent of the gas flow rate. This corresponds to the flow in the falling film region. In general the void fractions in downward flow were slightly higher than those with upward flow. The experimental data were compared with some correlations and data in the literature. Fairly good agreements (overall relative R.M.S. deviation of 18%) were obtained.