A versatile image analysis system which can be used to quantify the mixing pattern of a two-fluid/phase system has been developed. This visualisation technique, light absorption imaging (LAI), is based on the absorption of light by dyed fluids. In the context of this paper, the technique is applied to supercritical water hydrothermal synthesis (SWHS), which is used in the production of nano-particulate metal oxides. This case study was carried out in an attempt to highlight potential process pitfalls and to optimise overall reactor design. However, the versatility of this LAI technique makes the scope of possible applications much wider. The SWHS process for the production of metal oxide nano-particulates is a relatively new and promising development. A key aspect that requires fundamental research is the phase interaction or 'mixing mechanism' within these continuous reactors. The manner in which the supercritical water stream interacts with the cold metal salt stream is of paramount importance since it will inevitably affect the quality and quantity of product as well as highlighting any potential fundamental flaws in a design of the reactor. (C) 2004 Elsevier Ltd. All rights reserved.