The formation of shrinkage porosity is a concern in the production of high-quality gray iron castings. In this work, a geometry known to generate this type of defect was used to investigate some of the parameters that influence its formation. The geometry is based on the presence of a migrating hot spot that at the end of the solidification is located close to the interface between the casting and the mold. The occurrence of shrinkage porosity at this position was investigated and the cavities examined using a scanning electron microscope equipped with EDS. It is believed that this type of defect is in contact with the atmosphere during solidification. The risk for shrinkage porosity decreases with increasing carbon content. The effect of high levels of molybdenum and phosphorus was investigated and shown to influence the defect formation. Inoculation is used to control the nucleation and the effect of high levels of inoculants was also examined. The microstructure was investigated by the use of a color etching technique, and the quantification considered eutectic cell size and secondary dendrite arm spacing. The quantification was done on the microstructure in the vicinity of defects as well as in areas without porosity.