The characteristics of the spontaneous movement of condensate drops when a bulk temperature gradient is applied to a horizontal condensing surface in Marangoni dropwise condensation of a water-ethanol vapor mixture were experimentally investigated over a wide range of bulk temperature gradients and for various mass fractions. Drops were observed to move from the low-temperature side to the high-temperature side of the heat transfer surface. When the initial drop distance was adopted as a parameter for the Marangoni force acting on the condensate drop together with the surface tension gradient corresponding to the surface temperature of the condensing surface, the drop velocity was highly correlated with both the surface tension gradient and the initial drop distance over a wide range of parameters. At relatively large initial drop distances, the condensate drop velocity increases as the initial drop distance is reduced and it subsequently decreases after the velocity reaches its maximum value under an almost constant bulk surface tension gradient. The drop velocity increases linearly with increasing bulk surface tension gradient for a constant initial drop distance. (C) 2011 Elsevier Ltd. All rights reserved.