An in situ ATR-IR spectroscopic cell suitable for studies at solid-liquid interface is described including the design and experimental details in continuous flow mode at elevated temperatures (230 °C) and pressures (30 bar). The design parameters considered include the cell geometry, the procedure to immobilize the catalyst on a cylindrical internal reflection element (IRE), as well as shape and material of choice for the IRE, optics configuration and temperature/pressure control. The hydrodynamics and concentration profiles in the cell were assessed via numerical simulations using incompressible Navier-Stokes equation and convection-diffusion model showing significant deviation from ideal plug flow. Experimental response times at ambient and elevated temperatures derived from ATR-IR spectra agree well with the simulations, demonstrating that the hydrodynamics of the reactor is accurately described. Homogeneous Pt/ZrO2 and AlO(OH) layers were obtained by spray-coating which are stable for at least 12 h during testing in 2 mL/min water flow. Preliminary spectra collected during aqueous phase reforming of hydroxyacetone demonstrated that the cell can be operated at 230 °C and 30 bar with simultaneous detection of IR-active species both in the liquid as well as adsorbed on the surface of the catalyst.