Carbon inoculation has no effect on magnesium alloys that do not contain aluminium. The hypothesis proposed in a recent article [Scripta Materialia 49 (2003) 1129] that segregation of carbon plays a major role in the grain refinement of magnesium alloys by carbon inoculation is inconsistent with many of the observed facts. The Al4C3 or Al–C–O hypothesis, which is supported by experimental observations, is still the most reasonable mechanism proposed to date for the grain refinement of magnesium alloys by carbon inoculation.