A high pressure optical cell is presented, which allows in situ optical measurements in the ammonothermal process under real reaction conditions up to 200 MPa and 650 degrees C. The application of this optical cell is not restricted to a particular ammonothermal route, but can be used for different ammonothermal variants. Many so far unknown parameters could be investigated through simple imaging (e.g. particle movement). Video optical and spectroscopic observations in the ultraviolet to near infrared range show light absorption of the chemical species in the reaction fluid. The analysis of the acquired spectra leads to the conclusion that different intermediates are present during the increase of the temperature. The measurements commensurate with solubility data available in the literature. (C) 2011 Elsevier B.V. All rights reserved.