The onset of Marangoni instabilities in binary partially miscible liquid-liquid systems was first explained by Perez de Ortiz and Sawistowski who formulated and solved the linear stability problem for these systems. In the present paper, the same linear stability problem is investigated but the solution is obtained in a way that leads to the presentation of the characteristic equation in a new analytical form, which can be analysed easily. The stability criterion obtained for stationary instability is the same as the one by Perez de Ortiz and Sawistowski but has been found to differ for the onset of oscillatory instabilities. Another important finding of the present work is that stationary and oscillatory instabilities cannot exist simultaneously. The stability criteria developed have been applied to six binary systems and comparisons of the results obtained are made against experimental observations available in the literature. (C) 2004 Elsevier Ltd. All rights reserved.