Display omitted •A flow regime map for CO2-ethanol at 40 °C and for pressures up to 90 bar is presented.•CO2-ethanol flow regimes correlate with thermodynamic phase equilibrium.•Taylor bubble size increases linearly with UG/UL and follow models for ambient flows.•Bubble size decreases by gas dissolution until equilibrium is reached in the channel.•Relative bubble to unit cell length correlates with UG/UL, similar to ambient flows. The hydrodynamics of gaseous and supercritical CO2-ethanol flows in microchannels under elevated pressure (45–90 bar) at 40 °C have been visualized experimentally. The effect of pressure and CO2-ethanol mixture composition on the flow regimes developed has been investigated and related to the thermodynamic phase equilibrium diagram. The characteristics of gas-liquid Taylor flow formed at elevated pressure have been analyzed and compared with Taylor flows at ambient conditions. The results of this study show that depending on the pressure and mixture composition, different flow regimes are formed, namely two-phase Taylor flow, dissolving Taylor flow, single-phase jetting-dissolving flow and single-phase supercritical jetting flow. The formation of these flows is explained with respect to the phase equilibrium diagram of CO2-ethanol. In the two-phase flow region, Taylor bubble size has been shown to depend on both CO2-ethanol mixture composition and operating pressure due to changes in CO2 mass flow rate and density, respectively. The bubble size formed has shown to increase linearly with UG/UL closely following the well-known bubble formation models for ambient conditions. Due to the non-negligible solubility of CO2 in ethanol, the Taylor bubble size decreases in the channel until equilibrium is reached. Mass transfer is not easily quantified however due to the mutually soluble nature of CO2 and ethanol and the dependency of this solubility on pressure.