The vapour pressures of various salts have been investigated at high temperatures and the results are presented here together with the values found by previous investigators. Sodium sulphate is extensively used in the glass producing industry and it is in this compound that interest is centred. Other salts have been included in this study for the purpose of comparison. The use made of sodium sulphate in glass making is discussed briefly below. Work previously reported on the determination of the vapour pressure of sodium sulphate will be shown to have produced confusing results, and it was for the purpose of introducing some harmony into the sometimes discordant evidence that the present work was undertaken.