The microstructure of active carbon used in the growth of a silicon carbide catalyst support has been studied by BET area measurements, electron diffraction and high-resolution electron microscopy. The microstructure of the active carbon is consistent with different distributions of basic structural units (BSUs). These BSUs increase in size under heat treatment by forming adjacent distorted columns. Ordering processes are discussed from geometrical considerations based on reciprocal patterns. The reactivity of the interface between the active carbon and SiO vapour is controlled by the proportion of the highly reactive sites in the active carbon surface structure.