In this study, the hydrodynamic behavior of coflowing fluids CO2 and ethanol has been investigated in a high-pressure microfluidic reactor working at supercritical conditions, in which the two fluids are completely miscible. The velocity field has been measured by Micro Particle Image Velocimetry (PIV) for different temperatures between 20 and 50 °C at a fixed pressure of 100 bar. Meanwhile, we have developed a model to investigate numerically the mixing. By comparing the experimental results to a three-dimensional numerical simulation, the mixing model has been validated for the laminar coflow in the micromixer. In order to understand the mixing condition effects, several parameters have been investigated, namely: the Reynolds number, the temperature and the CO2/ethanol ratio. A mixing time constant is defined by using the segregation intensity curve and later used to characterize the mixing quality. The characteristic mixing time has been related to the laminar energy dissipation rate ∊, similarly to the stretching efficiency model in previous studies. The mixing quality is eventually analyzed in term of segregation index and mixing time.