The aim of this paper was to calculate the True Coincidence Summing Correction Factors (TSCFs) for an HPGe coaxial detector in order to correct the summing effect as a result of the presence of Y-88 and Co-60 in a multigamma source used to obtain a calibration efficiency curve. Results were obtained for three volumetric sources using the Monte Carlo toolkit, GEANT4. The first part of this paper deals with modeling the detector in order to obtain a simulated full energy peak efficiency curve. A quantitative comparison between the measured and simulated values was made across the entire energy range under study. The True Summing Correction Factors were calculated for Y-88 and Co-60 using the full peak efficiencies obtained with GEANT4. This methodology was subsequently applied to Cs-134, and presented a complex decay scheme. (C) 2016 Elsevier Ltd. All rights reserved.