A method is described for measuring the flux of a diffusing species through a multiple cell structure cut from a catalytic monolith honeycomb. One- and two-dimensional mathematical models are used to calculate the effective diffusivity in the catalyst/washcoat layer. This method is suitable for porous monolith supports, e.g. cordierite, but it is unsuitable for metal monoliths. To illustrate the technique the diffusion of CO in nitrogen is studied using a modified form of a Wicke-Kallenbach type of diffusion cell. The inlet concentration of the diffusing component is 2.4% CO in nitrogen, and experiments are performed at ambient temperature and pressures between 106 and 150 kPa on a catalytic monolith with 62 cells cm(-2). The technique can be applied in many areas where catalytic monoliths are used, e.g. catalytic converters, catalytic combustion reactors, SCR catalysts and many other applications. The method shows good agreement with the results obtained using other methods.