The development of diagnostic instrumentation for monitoring coal liquefaction process streams is discussed. A sapphire optical fiber was used as an attenuated total reflectance (ATR) element in conjunction with Fourier transform infrared (FT-IR) spectrometry to probe harsh liquefaction process streams. ATR provides a short, reproducible pathlength which allows for the analysis of highly absorbing materials, such as liquid hydrocarbons, and the properties of sapphire are well suited for the analysis of high temperature and high pressure process streams. A test cell was constructed which allowed in-situ monitoring of coal liquefaction reactions at 400 degree(s)C and 3000 psig. The cell incorporated a sapphire optical fiber as an ATR sensing element which was coupled to an FT-IR spectrometer using zirconium fluoride fiber optic cables. The spectra provide qualitative information about the liquefaction process.