A visualization study of pool boiling at atmospheric pressure from top-covered enhanced structures was conducted for a dielectric fluorocarbon liquid (PF 5060). The single layer enhanced structures studied were fabricated in copper and quartz, had an overall size of 10 mm x 10 mm and were 1 mm thick. The parameters investigated in this study were the heat flux (in the range of 1-11 W/cm(2) for copper and 1-4 W/cm(2) for quartz) and the width of the microchannels (65-360 mu m). A high-speed camera (maximum frame rate 1000 Us at full resolution) with attached magnifying lens allowed precise observation of the evaporation process in the bottom and top channels. The heat transfer performance of the enhanced structures was found to depend weakly on the channel width. The internal evaporation has a significant contribution to the total heat dissipation, especially at low heat fluxes. (c) 2005 Elsevier Ltd. All rights reserved.