This paper presents a method for IR spectroscopic measurement of sample under high temperature-pressure in situ. It comprises three parts: the high temperature-pressure IR spectroscopic cell; the composite recycle reactor-infrared spectrometer; application of these devices for measurement of active intermediates such as HCo(CO)4 and HCo(CO)3PBu3, which exist only under high temperatures and pressures. This technique is adaptable for investigation of changes of active intermediates, the sequence of elementary reactions, and the reaction mechanism.