The factors influencing the morphology of boron prepared by chemical vapour deposition in a closed system were investigated. The formation of crystalline or amorphous deposits is confined to well-separated regions defined by temperature and deposition rate or supersaturation. The experimental conditions for simultaneous growth of crystalline and amorphous boron in the temperature range 1390–1640 K are given by k = 1.59 × 10 8 × exp(−20 070/ T) where k is the deposition rate ( μg cm −2 s −1) and T the temperature (K). At deposition rates higher than that given by this relation, amorphous boron is formed. At temperatures lower than 1390 K, amorphous boron is formed at considerably lower deposition rates than those given by the relation. In the temperature range 1390−1640 K the transition of amorphous boron to crystalline boron is controlled by grain boundary self-diffusion with an activation energy of 172 kJ mol −1. Heat treatment of amorphous boron in vacuum in the temperature range 1270–1520 K results in the formation of β-rhombohedral boron via the formation and subsequent decomposition of α-rhombohedral boron. The crystals of α-rhombohedral boron are probably formed by a sublimation/ condensation mechanism.