The heat transfer limit of two control metal fit wicks and two step-graded metal felt, wicks was measured and compared to theoretical values. The wicks were planar and fabricated from 316 stainless steel. The working fluid was both water and methanol. The non-graded control wicks were measured to be at or near the wick's capillary heat transfer limit. The step-graded wicks failed much earlier than the theoretical capillary limit for each wick. Additional tests and analyses were conducted determining the failure to be caused from vapor formation within the step-graded wicks whereas the fine top layer trapped the vapor, resulting in failure. (C) 2004 Elsevier Ltd. All rights reserved.