This paper irìvestigates the effect of three different Ni additions (0.5, 1 and 2 wt%) on the high-temperature tensile properties of the commercial purity unmodified A356 (Al-7 wt% Si-0.3 wt% Mg) aluminium foundry alloy As-cast and T6 heat-treated tensile samples, obtained by permanent mould casting, have been tested at 508 K (235 °C). Fracture profiles and surfaces have been observed via optical and scanning electron microscopy to analyse the failure mechanism. It was observed that Ni contents higher than 0.5 wt% cause a remarkable decrease in yield strength and ultimate tensile strength of the T6 heat-treated alloys, which therefore become comparable with the tensile properties of the corresponding as-cast alloys. According to microstructural observations, the early rupture of fragile Ni-rich intermetallic phases seems to govern the fracture process of these alloys, neutralising the positive effect of the heat treatment.