Dispersion stability of colloids has been investigated in sub- and supercritical water by measuring the hydrodynamic diffusion coefficients of the particles by means of dynamic light scattering. It is interestingly found that coagulation of the colloids in sub- and supercritical water is a universal phenomenon irrespective of the material of the colloids. Highly charged colloids were found to be more stable in water against high temperature. Numerical analysis reveals that the stability of the colloids at elevated temperature and pressure is primarily governed by the temperature dependence of the dielectric constant of the medium. The effect of the temperature dependence of the ion product of water (pK(w)) was found to be very little. Surface charge density and Stern potential may change with respect to temperature due to the readjustment of the ion concentration in the diffuse layer through the enhanced ion product and reduced dielectric constant of water. These are the secondary causes of the particle coagulations in sub- and supercritical water.