Hydroboration of three allotropes of carbon, i.e., diamond (100) surface, [60]fullerene, and single-wall carbon nanotubes (SWNTs), with borane (BH(3)) has been explored by means of quantum chemical calculations. The calculations predicted that the hydroboration of C(60) and the C(100)-2x1 surface occurs readily, whereas the hydroboration of the sidewall of an armchair (5,5) SWNT is thermoneutral with a barrier height of 11.5 kcal/mol. This suggests that sidewall hydroboration, if viable, would be highly reversible on the (5,5) SWNT. The as-hydroborated carbonous materials can be good starting points for further chemical modification and manipulation of these carbonous materials, given the abundant chemistry of organoboranes.