AbstractOscillatory baffled reactors have obtained increasing popularity over the last decades, due to their high mixing efficiency at low flow rates. Several studies were performed on the optimization of geometrical and operational parameters. Yet, a full overview about the interactions in between those parameters is still missing, which can be ascribed to the high number of geometrical and operational parameters that can be varied. In the present work, a central composite rotatable design was used to obtain an overview about the interactions in between the geometrical and operational parameters. Through 3D‐printing, reactors were printed with high accuracy, assuring exact evaluation of geometrical effects on the flow. With particle image velocimetry the flow was characterized for effective mixing and the corresponding flow regime. The data obtained shows that the established optimization guidelines do not yield optimal operational conditions. Consequently, a new dimensionless number, the so called acceleration ratio , was introduced to offer additional guidelines for efficient reactor design. Moreover, it was found that the classical oscillatory Reynolds number does not sufficiently characterize the flow regime. An alternative form was derived from the classical Reynolds number and verified by experimental data. Both, the limits of the newly introduced acceleration ratio and redefined oscillatory Reynolds number are in good accordance with CFD‐results.