Abstract: Metal surfaces are contacted with a solution of a process bath (1), for example a phosphating or pickling solution, which comprises several components in aqueous solution, the composition of the process bath (1) being maintained within a given range by addition of solutions or gases, in particular air. The profitability is improved considerably if the solution of the process bath (1) is circulated via a first line (2) and a circulating pump (3) and the solution to be added and/or the gas to be added is fed to the process bath where, as a consequence of the ending of the line (2), vigorous thorough mixing prevails, or if the solution to be added or the gas to be added is fed to a suction pump (4), the solution to be added or the gas to be added mixing with the circulated solution.