In this paper a three-phase model (liquid, solid and gas) of the mushy zone for investigation of porosity formation in solidifying castings is developed. This model takes into account the release of the dissolved gas from the alloy as well as heat transfer and interdendritic fluid flow in the mushy zone. An important aspect of this work is related to accounting for the term describing porosity formation in the continuity equation. The magnitude and influence of this term is estimated. Distributions of residual porosity in castings are calculated based on a uniform solidification assumption for the particular case of aluminum-rich Al-Cu castings. An analytical criterion, identifying conditions under which there will be no porosity formation, is established.