It has been found that industrially important 4,4 '-dimethylbiphenyl can be produced with a maximum of 87% selectivity by the reaction between 4-methylbiphenyl and supercritical methanol over SAPO-11 catalyst at 573 K and 16.3 MPa. The reaction was investigated as a function of the zeolite type, system temperature, and loading of reactants. In addition, we developed a unique high-temperature/pressure optical cell with a diamond attenuated total reflectance element to measure the infrared spectra of reactive intermediates on the catalyst surface under our reaction conditions. This spectral technique could open the window into industrial chemical reactors.