Gas and liquid flow between the various compartments of surface treatment reactors have strong influences on the control of the reactant composition and temperature and, consequently, on the performance of the reactor. Two industrial examples are presented: the tracing of a multi-bath continuous pickling reactor to determine the amount of back mixing and its consequences on productivity, the tracing of multi-compartment annealing reactors to evaluate the gas exchanges between compartments. Mathematical modelling describes the observed behaviours.