This paper presents the experimental results on the effect of temperature on the void fraction and spatial evolution of the structure of air/water flows. The experimental techniques are based on measuring void fraction using gamma ray attenuation together with video images taken from high speed video recordings. The independent variables are: liquid and gas flow rates, temperature and axial position in the flow direction. The results show the significant effect of temperature on bubble size and shape as well as on void fraction. Bubble size and void fraction increase monotonically with temperature as a result of the partial saturation of air bubbles with water vapour. There are also changes in flow regime and a reduction of bubble clustering at elevated temperatures. This phenomenon becomes more important as the system approaches its boiling point.