The effect of the small fraction of clays on the rheological behaviour of alpine debris flow is poorly understood. This is partly due to the complexity of the debris flow mineralogy and the broad particle size distribution. This study has investigated this issue by simulating an alpine debris flow with a mixture of well characterized fractions and then varying the clay fraction composition. Four samples were tested, ranging from a clay fraction made up of only kaolinite (1:1 type clay) to samples where 80 per cent of the kaolinite is replaced by bedeillite (a 2:1 type clay similar to smectite). Changing the content of 2:1 type clay has a strong influence on the behaviour of the whole material, despite its low weight fraction of around 2 per cent. The tests carried out on these reconstituted materials were compared with the results obtained for natural debris flow materials and showed some common trends:in particular, the rheological parameters for materials with and without 2:1 clays with respect to yield stress as a function of solid content.