Experiments have been conducted in a square channel to investigate wall nucleation characteristics in subcooled boiling flows. The experimental data is presented as a parametric study of the effects of system pressures, flow rates and inlet subcoolings on nucleation characteristics. Bubble departure diameters and departure frequencies are measured and compared to an existing model. The model is shown to predict the trend in departure diameter well for all conditions and to predict departure frequencies well for all conditions except for those at low boiling number for low and moderate pressures. These conditions did not appear to be fully within the nucleating regime. Additionally, experiments to determine boiling heat transfer rate and critical heat flux (CHF) at subatmospheric pressures were conducted. Heat transfer and CHF is shown to be significantly lower under subatmospheric conditions.