A flow cell fitted with large diameter optical ports suitable for visual observation and Raman spectroscopic studies of aqueous solutions to temperatures of 500 degreesC and pressures to 25 MPa is described. The cell uses commercially available compression fittings that have proven to be reliable and robust to cycles of assembly, temperature exposure, and disassembly. A unique design is given for construction of the optical ports. The design of a heated cylindrical insert for salt solubility, salt deposition and salt nucleation studies in near-critical aqueous solutions is described. A method for in situ detection of the crystalline phase of salt deposited from aqueous solution at near-critical conditions and then surrounded by pure supercritical water is demonstrated for Na2SO4, using Raman spectroscopic observation. (C) 2002 Elsevier Science B.V. All rights reserved.