Experimental results for the high pressure phase behaviour of fluid {xCF(4) + (1 - x)H2O} are presented. (Gas + gas) equilibria for 10 compositions, with (0.0121 +/- 0.0002) < x < (0.0583 +/- 0.0002), were measured in a high pressure autoclave with sapphire windows and magnetic stirring. Phase transitions were determined visually. The pressure range was from 20 MPa to 200 MPa and the temperature range from 580 K to 673 K. The {xCF(4) + (1 - x)H2O} system exhibits type-III fluid phase behaviour. For two mixtures, a critical point could be measured. The slope of the critical line commencing in the critical point of H2O starts with a negative slope. However, additional critical point measurements will be needed to establish (gas f gas) immiscibility of the second kind in {xCF(4) + (1 - x)H2O}. The objective of this study was the investigation of the phase behaviour of a model system encountered in supercritical water oxidation processes.