The phase behavior and reaction of nylon 6/6 in water were studied with a diamond anvil cell (DAC) technique and visual microscopy. Nylon 6/6 concentrations in water and cell temperatures were varied from 11 to 46% and from 264 to 425 degrees C, respectively. The pressures studied ranged from 30 to 900 MPa. When an aqueous solution of 27% nylon 6/6 was rapidly heated (2.6 degrees C/s) to 372 degrees C at 30 MPa, the solution became homogeneous at 331 degrees C. Upon cooling, the final pressure was 30 MPa and both particles and gas were observed, Analysis of the particles by Raman indicated decomposed nylon 6/6 solid. When an aqueous solution of 31% nylon 6/6 was rapidly heated (2.9 degrees C/s) to 425 degrees C at 58 MPa, the solution became homogeneous at 323 degrees C. Upon cooling, the final pressure was 143 MPa, and, remarkably, only a second Liquid precipitated and no gas or solids were observed. From the experiments, we concluded that the reaction pathways are completely different between the subcritical and supercritical water conditions. For the case of subcritical conditions, the final products were solid particles having a nylon character along with a considerable amount of gas. At supercritical water conditions, the final products were liquids having little nylon character and no gas. Experiments were performed at, a constant temperature of 272 degrees C at initial pressures ranging from 87 to 400 MPa. As the reaction proceeded, the pressure was measured at 30-s intervals. At average pressures less than 300 MPa, the nylon 6/6 samples melted and appeared to become homogeneous, At average pressures higher than 520 MPa, the nylon 6/6 samples remained heterogeneous, From these results, the rate of hydrolysis was concluded to increase with pressure. The reaction volume was found to be -21.1 cm(3)/mol, which can be explained by the overall formation of water-soluble products. (C) 2000 John Wiley & Sons, Inc.