The Knoop microhardness anisotropy of triclinic potassium bichromate single crystals was investigated on the (001) cleaved face at two values of test load. 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 revealed a distinct anisotropy consisting of maxima and minima of microhardness for the < 100 > and < 110 > indenter orientations. Additionally, a fair anisotropy associated with differences between maxima for indenter oriented in the [110] and [1 (1) over bar0] directions and minima for the [010] and [100] directions was observed. It was found that the computed effective resolved shear stress based on the {110}(1 (1) over bar0) slip system explains the experimental results of the hardness anisotropy on the (001) face of potassium bichromate single crystals. (C) 2012 Elsevier B.V. All rights reserved.