Controlled adjustment of gas concentrations in laboratory recycle reactors, highly desirable to support kinetic studies on solid catalysts, seems often difficult to accomplish. Main obstacles are large time delays, strong loop interactions, and of course incomplete knowledge about the kinetics of the reaction system under investigation. To cope with such difficulties, a simple recursive I-control algorithm has been developed on the basis of a general control concept proposed some fifteen years ago. Simulation studies indicate good performance of this algorithm and robustness to uncertainties of model parameters.