The distribution of gas and liquid in gas-liquid two-phase slug flow was measured using semi-supermultiple point-electrode probes. Based on the measurements, the wake zone behind the gas slug and the low void fraction zone in the liquid slug are defined and the void fractions of the two zones are determined. The data reveals that the void fraction of the wake zone increases according to Jg, while there is little dependence. On jι. Non-dimensional head is proposed in consideration of the momentum change of the liquid in the wake zone. It is clarified that non-dimensional head is closely related to the void fraction of the wake zone. A good practical relationship is found between non-dimensional head and the lengths of the swelling liquid front zone and the wake zone, and empirical correlations are proposed for the void fraction in the wake zone, the mean void fraction in the liquid slug, and the lengths of the swelling liquid front zone and the wake zone.