Collecting spatially resolved gas-phase concentration profiles in catalytic monoliths by applying suction probe techniques has become an important tool for understanding the reaction sequence and for optimizing the design of structured catalysts. Here, the significance of the probing technique' simpact on the measured data is investigated by means of CFD simulations and experimental investigation of catalytic partial oxidation of CH4 on a Rh-coated honeycomb catalyst. Different positions of a suction probe (diameter = 170 mu m) inside a rectangular channel ( height = 795 mu m) of a monolith are compared with respect to the influence on flow field, residence time, and reaction progress. The influence of the probe on the measured data is shown to strongly depend on its position. The maximum error can be over 50% with respect to the residence time. For an isothermal process, the error in the concentration profiles can be corrected by using a function which is only dependent on the geometrical structure of the monolith. However, for a process with a temperature gradient along the catalytic channel, such as CPOX of methane, the correction function is not applicable. An appropriate comparison of experiments and simulations requires a three-dimensional CFD simulation taking the probe into account. (C) 2013 Elsevier B.V. All rights reserved.