A novel mid-infrared optical sensor enabling in situ ATR measurements was applied to investigate several steps of a vitamin D analogue synthesis. The probe based on silver halide fibers coupled to a diamond prism was connected to a conventional FTIR spectrometer with internal MCT detector. All steps of the reaction were monitored by real-time in situ FTIR measurements. The steps carried out were the dissolution of SO2 in a CH2Cl2/CH3OH solvent mixture as well as the addition of SO2 to a vitamin D analogue, the subsequent ozonation of the double bond in the SO2 addition product, and the following reduction of the formed hydroperoxide with triphenylphosphine. The dissolving process of SO2 and the addition of SO2 to the vitamin D analogue were monitored by changing the characteristic v(as)(SO2) and v(s)(SO2) modes of dissolved and incorporated SO2. It was found that during ozonation of the SO2 addition product the formation of hydroperoxide is accompanied by the simultaneous formation of the corresponding aldehyde identified by the typical v(C=O) band at 1720 cm(-1). Extended ozone exposure favors the formation of the corresponding acid detected by an additional carbonyl band at lower wavenumbers. During the reaction with triphenylphosphine the increasing intensity of the aldehyde band and the appearance of the v(P=O) mode of the formed triphenylphosphine oxide indicate the progressive reduction of hydroperoxide. The hydroperoxide band disappears completely during the reaction whereas the v(as)(SO2) band remains unaffected.