The Knoop microhardness anisotropy of monoclinic gadolinium calcium oxyborate single crystals was investigated on the (001), (100) and (010) faces. The experimental results of the microhardness anisotropy were analysed for different slip systems using a model based on the effective resolved shear stress. The results obtained for all three faces revealed hardness anisotropy consisting of one maximum or minimum in the 180 degrees range of indenter orientation. From experimental data and computed effective resolved shear stress it was found that the {101} and {011} slip planes are active during the indentation deformation of all three faces whereas the {110} slip planes are also active during the indentation of the (001) and (100) faces of gadolinium calcium oxyborate crystals. (C) 2014 Elsevier B.V. All rights reserved.