The relationship between the cooling curve and formation of primary silicon from the melt has been investigated for Al–20%Si alloy in the uninoculated state and when prior-treated with a phosphorus-bearing inoculant. The results suggest that coarse branched silicon forms from an uninoculated melt at sub-liquidus temperatures well above the primary silicon arrest temperature and that increasingly effective inoculation both reduces intensity of the arrest and shifts it to higher temperature eventually eliminating it completely in favour of a sharp reduction in slope of the cooling curve at or just below the liquidus temperature. It is proposed that this could be used as a simple diagnostic test for efficiency of refinement of primary silicon by inoculant additions under specified conditions.