During the production of aluminium ingots and castings, the surface oxide on the liquid may be folded into the bulk liquid to produce crack-like defects (bifilms) that are extremely thin, but can be extensive, and so constitute seriously detrimental defects. In this work, it has been found that bifilms are the initiator and hydrogen is only a contributor to the porosity formation process. For the first time, evidence is presented for the contribution of air ( or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The discriminating use of the RPT clearly reveals the existence of bifilms, and the effect of hydrogen on porosity formation. However, it seems that the RPT is of little use to evaluate the hydrogen content of the alloy. To investigate these effects, two alloys were studied in laboratory experiments LM0 (99.5% Al), LM4 (Al-5Si-3Cu) and one in an industrial environment LM24 (Al-8Si-3Cu-Fe).