An experimental work, aimed at appraising the authors' theoretical analysis of the simultaneous film evaporation outside and condensation inside horizontal tubes, is presented. The data indicate that for ReN > 150, the laminar flow model must be modified to account for ripples and waves caused by the drops falling down on the tubes. The effects of ReNand ΔT on the overall transfer coefficient are very small. The effect of Tυ is small, particularly in the presence of air. The predicted effects of utilizing non-circular conduits is validated, and a 20% increase in the overall transfer coefficient was measured for elongated elliptical conduits.