Concentrations in recycle reactors and continuous stirred tank reactors can be controlled by a computer based system using a simple algorithm in which kinetic orders of zero are assumed for all reactants. The only control parameter is the interval time at which new flow rates are set. The algorithm has been successfully used to control five concentrations during an investigation of methanol reforming over a copper catalyst. Simulations, including an autocatalytic reaction and delays in the analysis, prove the general applicability. A broad optimum exists for interval times of 0.5-1 times the dimensionless time (theta=t/tau) leading to a steady-state value within 8-14 theta. The performance of this new approach is comparable to more sophisticated concepts except the most recent model-based approaches but requires only minimal effort and may thus lead to a wider use of concentration controlling in kinetic studies.