Leaves of two plant species eaten by Macaca fascicularis in Bukit Timah Nature Reserve, Singapore, were collected and colour-tested. Leaves matching those eaten by M. fascicularis were examined by energy-dispersive X-ray micro-analysis. The leaves of Streblus elongatus (Moraceae) and Gluta wallichii (Anacardiaceae), together forming 19.6% of the leaf diet of the macaques, contained silica. In G. wallichii, this is in the base of hairs that project from the underside of the leaf, whereas S. elongatus leaves have short sharp siliceous trichomes which are densely packed on the undersurface of leaf veins. We predict from an indentation analysis that chewing on the latter species could cause dental microwear at low occlusal forces. The leaves are reportedly common in the diet of three other primate species in peninsular Malaysia and the finding could have general significance for studies of dental wear.