Mow experiments simulating the rapid precipitation of salts during the supercritical water oxidation (SCWO) waste treatment process were performed. Aqueous salt solutions were injected into a coaxially flowing supercritical water Stream at a constant pressure of 250 bar. Jet concentrations ranged from 0.1 to 10.0 wt % salt with a typical flow rate of 0.5 g min(-1) and temperature of 150 degrees C. The flow rate of the pure supercritical water stream was typically 10.2 g min(-1) with an initial temperature of 550 degrees C. Results from scanning electron microscopy of collected solids, in situ laser transmission measurements, and low-magnification microscopic or visual observation of the jets indicated that, at 250 bar, sodium chloride solutions first pass through a two-phase, vapor-liquid state before solid salt is formed, while sodium sulfate solutions nucleate solids directly from a homogeneous supercritical-fluid phase. Sodium sulfate solids appeared much finer and also more aggregated than sodium chloride solids. Primary sodium sulfate particle diameters were typically between 1 and 3 mu m, while some aggregates reached diameters up to about 20 mu m. In contrast, sodium chloride solids ranged from 5- to 25-mu m shell-like particles for a 0.5 wt % NaCl jet and 20- to 100-mu m semispherical particles for a 10.0 wt % NaCl jet. At a subcritical pressure of 200 bar, the average particle size increased dramatically for both salts. In mixed NaCl/Na2SO4 solutions at 250 bar, the extent of small particle nucleation of sodium sulfate decreased with increasing sodium chloride concentration in the jet feed. Both the observed morphology and mixture effects were explained in terms of different isobaric phase behavior.