The dynamic air saturation method of measuring vapour pressures has been applied to sulphuric acid solutions at 25°C. The partial water vapour pressures have been determined in the range 45 - 95% acid, thereby confirming and extending previous results obtained by direct static measurements. The application of a radioactive tracer technique to the air saturation method of determining vapour pressures has been successfully accomplished. The calculated activities obtained in the more concentrated acid region were found to be in moderate agreement with previous values obtained from indirect measurements. A new method of tritium assay involving -phase gas counting of tritiated alcohol has been developed and substantiated.