The behavior of alkali salts in a continuously stirred tank reactor is of particular interest with respect to the development of the hydrothermal biomass gasification process. On the one hand, alkali salts increase gas formation. On the other hand, they may plug reactors due to precipitation. To obtain information oil the potential enrichment of salts in continuously stirred tank reactors, cumulative residence time distribution functions were measured using K2CO3 and NaHCO3 tracers and phenol as a reference.