A comparison between flow of air-water and steam-water through a venturi has established key differences between the two systems. The variations of local void comparison between the flow of air-water and steam-water mixtures through a venturi fraction of two-phase flows through a converging-diverging venturi have been studied using gamma ray attenuation. The independent variables are the liquid and gas flow rates, the temperature and the axial position in the flow direction. The difference between the two systems is found to be most evident at, and downstream of the throat. In steam-water, void fraction increases substantially at, and beyond the throat as a result of the low local pressure at the throat. On the other hand, with air-water the void fraction at the throat decreases at low temperature because of local changes in the slip ratio of the two phases. The effect of temperature on flow through a venturi is found to be more significant than it is in a straight pipe. This phenomenon becomes more important as the system approaches its boiling point.