Measurements of hardness anisotropy by Knoop diamond indentation on the {100} surfaces of Nb6C5 crystals show that the hardness is determined by crystallographic slip on {111} 〈1¯10〉 and {110} 〈1¯10〉 systems. {111} is the preferred slip plane for Nb6C5 and crystals with higher carbon content which show a marked decrease in Knoop hardness. The carbon atom/vacancy arrangement in these crystals is shown, by electron diffraction, to possess short-range order. Crystals annealed at low temperatures contain domains of non-cubic long-range order which increase the Knoop hardness and eliminate the anisotropy in hardness. Dislocation arrangements around Knoop indentations have been directly observed by electron microscopy in an attempt to confirm the slip processes deduced from hardness anisotropy.