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A novel technique for measuring the axial distribution of wall flow in diesel particle filters is described. It is based on measuring the dilution of a tracer gas (introduced at the upstream end of an outlet channel) as it passes along a filter channel, due to the wall flow. A relation is derived between the change in concentration and the flow per unit length of air entering the outlet channel from adjacent inlet channels. Typical data showing the axial variation in the wall flow for clean and loaded filters are presented and discussed. We find qualitatively plausible flow distributions, but note that independent verification of the method is required before the data can confidently be interpreted quantitatively. Some of the potential applications of the technique may include studying the regeneration process and measurement of ash deposition.