This paper presents the results from an experimental investigation of propane condensing inside horizontal tubes at three mass fluxes, two tube diameters and reduced pressures of 0.25-0.95. These conditions represent the saturation pressures and mass fluxes of most interest to the process and HVAC&R industries. In this first part, Part I, of a two-part study, the trends in heat transfer coefficient and frictional pressure gradient for these conditions are discussed and comparisons with available models from the literature are made. A small increase in heat transfer coefficient with tube diameter is observed for the specific tube diameters range studied, which is different from the trend predicted by most correlations. High-speed flow visualization results from a previous study, conducted using the same test facility, are used to explain the trend with tube diameter. No heat transfer coefficient correlation from the literature predicted all the tesults from the present study adequately; notably, all correlations perform increasingly poorly at higher reduced pressures. Also, no correlation from the literature adequately predicts the frictional pressure drops measured in this study. In the companion paper, Part II, the heat transfer and pressure drop results from this paper are used to develop correlations applicable to a wide range of operating conditions. (C) 2015 Elsevier Ltd. All rights reserved.