Continuous hydrothermal synthesis (CHTS) is a promising process for the synthesis of metal‐oxide nanoparticles. The process is based on the mixing of a cold aqueous metal salt solution with a near/supercritical water stream. Due to the exceptional properties of the supercritical water, particle formation occurs. In addition to the study of the influence of the process parameters on the resulting particle size and distribution, the residual concentration of dissolved metal salt is of interest. This can be analyzed by impedance spectroscopy. Therefore, preliminary tests are carried out on a modified CHTS plant to directly determine the residual concentration in the future.