Concentrated photovoltaic thermal systems (CPV/T) have an important place among solar energy systems because of converting solar energy directly into electrical energy and utilizing waste heat of photovoltaic modules. In the present study, the effect of preheating of the Rankine cycle (RC) heat source (flat plate solar collector) with the CPV/T system on the RC efficiency and hydrogen production was investigated. A theoretical investigation was carried out for different parameters as CPV module aperture area, direct solar radiation, concentration ratio, photovoltaic cell type, mass flow rate and evaporator saturation pressure. Also, the conditions with and without the CPV/T system were compared in terms of system efficiency and hydrogen production. The results of the analysis revealed that the CPV/T system increased energy/exergy efficiencies and hydrogen production at different rates for all parameter values. It was determined that the most important parameters affecting the energy/exergy efficiency and hydrogen production were the CPV module aperture area (A(PV)) and the concentrating ratio (C). With the use of CPV/T system, an increase in the production of hydrogen in the range of 0.02 kg/h to 0.30 kg/h depending on the parameters was achieved. In addition, exergy destruction rates calculated for the integrated system components and the highest exergy destruction rates were determined to occur in the flat plate solar collector (FPSC) and CPV/T system. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.