Ce1 − xZrxO2 solid solutions are produced continuously by hydrolysis of mixtures of cerium ammonium nitrate and zirconium acetate in near-critical water at ca. 300 °C and 25 MPa using a flow reactor. Rapid hydrothermal coprecipitation leads to nano-particulate Ce1 − xZrxO2 (x = 0–1), the composition of which is largely determined by the initial relative concentrations of Ce4+ and Zr4+ ions in the starting solution. The freshly prepared materials are crystalline, possess very small particle sizes, and have high surface areas. The effects of calcining the products in air at high temperature have been studied. Apart from the 1∶1 Ce∶Zr solid solution, the phases of Ce1 − xZrxO2 remain stable on calcining to 1000 °C, but the particles sinter and the surface areas decrease significantly. The materials have been characterised by Powder X-ray Diffraction (PXD), IR and Raman Spectroscopy, microanalysis, Thermogravimetric Analysis (TGA), X-ray Fluorescence (XRF) and BET (surface area measurements). In selected cases, high resolution Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images were recorded to examine the particle morphology.