In this work, an experimental study was made on gas injection into an oil-water flow in horizontal pipes with two unequal pipe diameters. Special attention was given to the influence of gas injection on the average in-situ oil fraction. Measurements were made for input water flow rates of 1.25-5 m(3)/h, input oil flow rates of 0-8 m(3)/h and input gas flow rates of 0-9 m(3)/h. It was found that gas injection has a considerable influence on the in-situ oil fraction. In general, a small increase in the rate of air injection leads to greatly decreasing in-situ oil fractions. The in-situ oil fraction with gas injection decreases to a greater extent than that without gas injection, at the same input liquid flow rates. At a given input water flow rate, the value of the in-situ oil fraction in the pipe with the larger diameter is higher than that in the pipe with the smaller diameter. Furthermore, the drift flux models were extended to predict the average in-situ fractions of the oil phase in the intermittent three-phase flow regimes. A good agreement is obtained between theory and data, especially for the in-situ oil fraction range of 0.2-1.0.