The solidification path of a dilute Al-Cu alloy was studied using controlled solidification conditions and thermal analysis. Under equilibrium considerations, below the limit of maximum solubility, a unique alpha phase is expected, rounded by rich non eutectic composition. However, the precipitation of the second phase theta is present even for dilute compositions, fundamentally favored by segregation in the liquid and instabilities in the front of solidification. This effect has technological and academic implications, related to the precipitation of intermetallic compounds from the melt.