The effect of uniform external heat flux on the effectiveness of counterflow microchannel heat exchangers is experimentally studied in this article for validating an existing thermal model. The model validated in this study is a one-dimensional model previously developed by the same authors. The model is validated to be independent of microchannel profile, hydraulic diameter, and heat capacity ratio. For studying the effect of microchannel profile, experiments are conducted under balanced flow conditions using trapezoidal and triangular microchannels with approximately equal hydraulic diameter of 278.5 mu m and 279.8 mu m, respectively. The influence of hydraulic diameter on the thermal model is studied using a trapezoidal microchannel with hydraulic diameter of 231 mu m and 278.5 mu m. Experiments are conducted under unbalanced flow conditions, with a heat capacity ratio of 0.5, using the trapezoidal microchannel of hydraulic diameter of 278.5 mu m. Deionized water is used as the fluid in all experiments. The hot and cold fluid effectiveness is studied and the theoretical predictions and experimental results are found to be in excellent agreement. Thus, the model validated in this article can be used for accurately modeling microchannel heat exchangers irrespective of the microchannel hydraulic diameter, profile, and heat capacity ratio.