The flexibility of slave labour as an economic institution has often been assumed as a given. In general, some capital investment is necessary to retrain novice slaves but essentially they could be substituted for any other form of labour. This paper refutes the claim of the flexibility of slave labour through employing a longitudinal study for the Graaff-Reinet region of the Cape colony. We calculate Hicksian elasticity of complementarity coefficients for each year of a 21-year combination of cross-sectional tax datasets (1805-28) in order to test whether slave labour was substitutable with other forms of labour. We find that khoe, family and slave labour are not substitutable over the period of the study. This lends credence to the finding that slave and settler family labour were two different inputs in the agricultural production process. Indigenous Khoe and slave labour also remain complements throughout the period of study even when Khoe labour becomes scarce after the frontier conflicts, confirming the notion that slave labour at Graaff-Reinet was not a flexible labour source. We argue that the lack of substitutability of slave labour was due to the need of the settlers to acquire labour with location-specific skills such as the indigenous Khoe.