A mechanism is postulated to account for the exchange of electrical charge that has been observed to take place when hydrometeors in a cloud created in the laboratory collide. A hydrodynamic flow induced by variation of the surface tension at an air/water interface is regarded as the main driving force. This flow created by forces acting within an interfacial region will transport the excess negative charge that normally resides at an air/water interface from one interfacial region to another, thereby yielding a separation of charge. An estimate of the quantity of charge exchanged by this process, using reasonable parametric values, is in accord with laboratory observations. In some interactions our proposed mechanism is only valid if a stable thick water film coats the surface of one of the colliding hydrometeors. Measurements are presented to show that the ice/water interface is negative, and this is a condition needed to confer stability of thick water films on ice. ¿ 2001 American Geophysical Union