This communication documents the experimental investigation of the theoretical model for predicting the thermal performance of parallel flow microchannel heat exchangers subjected to external heat flux. The thermal model investigated in this communication is that previously developed by the authors of this communication; Mathew and Hegab [B. Mathew, H. Hegab, Application of effectiveness-NTU relationship to parallel flowmicrochannel heat exchangers subjected to external heat transfer, International journal of Thermal Sciences 31 (2010) 76-85]. The validity of the theoretical model with respect to microchannel profile, hydraulic diameter, heat capacity ratio and degree of external heat transfer is checked. The microchannel profiles investigated are trapezoidal and triangular with hydraulic diameter of 278.5 and 279.5 mu m, respectively. The influence of hydraulic diameter is analyzed using trapezoidal microchannels with hydraulic diameters of 231 and 278.5 mu m. Experiments are conducted for heat capacity ratios of unity and 0.5 using the heat exchanger employing the trapezoidal microchannel with hydraulic diameter of 278.5 mu m for purposes of validating the model. Experiments are done for all heat exchangers for two different levels of external heat transfer; 15% and 30% of the maximum possible heat transfer. Irrespective of the parameter that is investigated the experimental data are found to perfectly match with the theoretical predictions thereby validating the thermal model investigated in this communication. (C) 2011 Elsevier Ltd. All rights reserved.