The reliable prediction of pressure drop in two-phase flow is an important prerequisite to accurate optimization of thermal systems. Because of the complexity of these types of flow, empirical or semiempirical relationships are only of limited reliability and pressure drops predicted using leading methods often differ by 50% or more. In response to this situation, this work presents an experimental (Part I) and analytical (Part II) investigation of two-phase pressure drops during evaporation in horizontal tubes. The goal of the experimental part was to obtain accurate two-phase pressure drop data over a wide range of experimental conditions. The experimental conditions were chosen so that the effect of the principal parameters could be identified. The range of experimental conditions covered were: three refrigerants (low, medium and high pressure, respectively), two internal tube diameters, eight mass velocities and four heat fluxes (plus adiabatic flow). In addition, using water as the heating fluid, it was possible to cover the entire range of vapor quality. The two-zone test sections allowed tests to (i) be run under both diabatic and adiabatic conditions simultaneously, (ii) to obtain two-phase pressure drop values for nearly every flow regime, and (iii) to validate the data reduction procedure used to obtain the frictional component of the pressure drop. The campaign acquired 2543 experimental two-phase pressure drop values covering five flow regimes.