Visualization experiments are conducted to explore the condensation process in sintered copper wicks of operating flat-plate heat pipes. Two working fluids, water and methanol, and two wick types, mesh and powder wicks, are investigated. The effect of non-condensable gas has been carefully controlled at an insignificant level. The steady-state condenser resistances R-e and the evaporator resistances R-e are measured at incremented heat loads Q. For water, cyclic dropwise condensation is observed on exposed wick tops due to the finite water/copper contact angle. For methanol, the condenser is submerged in the liquid as a result of zero contact angle and ineffective liquid delivery out of the condenser. This leads to quiescent methanol condensation with higher R(c)s. For all the tests, R(e)s (in Kcm(2)/W) are several times larger than R(e)s. The main reasons are the thicker liquid layer in the condenser wick and the large thermal resistance associated with the condensed liquid layer, with the latter dominating. Considerable under-prediction of R-c may exist in current theoretical models which simply applies the conduction-limit model without considering the thermal resistance from the condensed-liquid layer. (C) 2013 Elsevier Ltd. All rights reserved.