Spray cooling experiments have been performed on a smooth heater surface. The working fluid water was atomized through a full cone nozzle into a low pressure spray chamber. Different coolant mass fluxes were studied by varying the distance between nozzle and heater. Observations obtained with an infrared camera indicated that the length of the three phase contact line increased as the heat flux in the experiments was raised. Motivated by this finding, the effect of different micro structured surfaces, which lead to an increase in the three phase contact line, on the spray cooling performance was studied. The micro structures consisted of micro pyramids with different heights. A significant enhancement in the heat transfer performance due to the surface structures could be observed, especially at low coolant fluxes. Additionally. high spatial resolution temperature measurements oil a smooth heater using thermochromic liquid crystals were obtained. These measurements indicate high local temperature gradients for a regime where the coolant film on the heater is ruptured. (C) 2007 Elsevier Ltd. All rights reserved.