A new apparatus has been made for the precise measurement of heats of mixing of liquids. With this apparatus measurements can be made in the absence of air, vapour, or mercury for systems which have volume changes on mixing of up to 1 ml per mol of mixture. The apparatus has been used to measure the heats of mixing of ethanol, 1-propanol, and 1-butanol with benzene at 45 �C for comparison with previous measurements. New measurements have also been made over the concentration range for heats of mixing of ethanol, 1-propanol, and 1-butanol with benzene at 25 and 35 �C and for methanol, 1-hexanol, and 1-octanol with benzene and ethanol with toluene at 35 �C. For these systems the heat of mixing per mol of mixture has a maximum at a mole fraction of alcohol from 0.30 to 0.35. The results show that the heats of mixing at a mole fraction of alcohol of 0.30 increase linearly with increase in temperature and at a given temperature they increase with the molecular weight of the alcohol but at a lower rate for the higher alcohols.