A set of 72 experiments was carried out to study the effects of solidification conditions, hydrogen content and additives on the formation of porosity in Al-9 wt% Si-3 wt% Cu-X alloy systems. It was found that not all alloying elements contribute to porosity formation in the Al-Si-Cu base system. Some of these elements, e.g. magnesium, titanium and phosphorus, tend to reduce both pore size and density. Hydrogen is the strongest element that induces porosity formation, its effect being reinforced either by the addition of strontium or by increasing the solidification time, or both. Grain refining is found to reduce pore density and pore size, and results in a fine dispersion of the pores throughout the alloy matrix. The necessary precautions to be taken in measuring the porosity in these alloys are reviewed in this paper. Accurate measurements of porosity using image analysis need careful adjustment of optical parameters, namely focus, illumination and grey level, as well as a careful selection of the number of field measurements required to represent correctly the sample surface.