The instabilities of an absorbing falling film in the presence of a surface-active agent are studied at low or moderate Reynolds numbers. This work is an extension of the previous stability analysis for a falling film in the presence of a surfactant (Ji and Setterwall, 1994, J. Fluid Mech. 278, 297-323). The present model considers the absorption of water vapour which includes both mass transfer and heat transfer. There are three unstable modes for an absorbing falling film (without surface-active agent) : one is the surface wave which is inherent to free surface flow, and the other two are associated with the Marangoni instability. One of these occurs for large positive absorption Marangoni numbers, being induced by the surface tension force resulting from the concentration gradient of water. The other one occurs for large negative absorption Marangoni numbers, and it is induced by the surface tension force resulting from the temperature gradient produced by the releasing heat of absorption. The effects of surface-active solute on these modes of instabilities are determined.