Hydroaminomethylation is an interesting synthesis route for manufacturing amines from olefins in one-pot reaction; in particular, primary and linear olefins. Reaction conditions of hydroaminomethylation concerning temperature and pressure using ammonia as amine source are in general close to supercritical state of ammonia. Hydroaminomethylation processing under supercritical conditions, in particular, is expected to offer some advantages such as a homogeneous reaction mixture, etc. In the present study, a high-pressure set-up for liquid ammonia operations containing an optical cell and a peripheral metering system was constructed. This set-up enables the observation of opalescence and opacity phenomena (critical point) of various hydroaminomethylation reactant/product mixtures at different temperatures and pressures (up to 180 degrees C and 300 bar). The critical points were achieved by stepwise increasing of temperature and pressure, which appear in a region from 92-290 bar and 120-172 degrees C. They were found to depend on the type of mixtures used (components, molar ratios, etc.). Furthermore, the measurements have shown that the influence of various olefins and co-solvents on the position of the critical points is rather small in the case of high ammonia to olefin molar ratio (ammonia:olefin > 25), whereas at lower ratios the differences are significant. The addition of strongly interacting compounds such as water significantly shifts critical pressures to higher values. This has to be considered in reaction runs because water is a reaction product. The system could be operated in a stable temperature regime under supercritical conditions at high excess ammonia and sufficient syngas amount. It can be assumed that the supercritical state will not be left during reaction runs at temperatures around 140 degrees C. No significant corrosion was observed during approximately 100 opalescence runs. (c) 2007 Elsevier B.V. All rights reserved.