When a liquid evaporates under vacuum, its free surface is potentially unstable to local variations in evaporative flux, surface depressions being produced by the recoil force of the departing vapour and sustained convection in the liquid being driven by the shearing action of the vapour on the distorted liquid surface. For a binary mixture, local variations in evaporative flux may be produced by fluctuations in both surface concentration and temperature. With the aid of linear hydrodynamic-stability theory, this paper examines the extents to which key mass-transfer properties affect the interfacial stability of the system. The mass-transfer aspects that distinguish this problem from its heat-transfer analogue centre on the dependence of relative volatility on temperature and composition as well as the importance of the bulk-flow term in Fick's law. Results indicate that the stability criteria for interfacial convection are extremely sensitive to the difference in volatility between the two components, that the destabilizing effects of surface concentration and temperature on evaporative flux are additive in determining stability limits, and that for certain operating pressures spontaneous convection can only be induced by adverse concentration gradients. Attention is limited to low-surface-tension mixtures for which there are no concentration effects on surface tension (Marangoni instability).