This paper presents results of an experimental study on the effect of suction on the shear behaviour of sand/silt mixtures as determined with direct shear tests performed under suction control. The device used to perform these tests is a direct shear apparatus in which the shear box has been replaced by an air pressure chamber where matric suction can be applied by increasing the air pressure in the cell chamber while the water pressure at the bottom of the sample remains atmospheric (axis translation technique). The samples have been initially consolidated under constant normal stresses between 100 and 900 kPa for different matric suctions ranging from 0 to 1200 kPa. Afterwards, while the normal stress and the matric suction are kept constant, the samples have been sheared. The results obtained in the shear tests show the expected increase in strength with suction, particularly in cohesion, although the increase is less significant towards high suctions and for the high vertical stresses. In the range of stresses tested the relation between suction and cohesion is linear. The friction angle with respect to suction is lower than the internal friction angle, and decreases with the level of suction until reaching a constant, low value for the suctions corresponding to the residual zone of the water retention curve.