Metallurgical factors of cast iron modification are the core of both theoretical and productive problems in the research field of cast iron. Having developed methods to analyse active oxygen and sulfur in the melts, the authors quantitatively examined their anti-nodularizing functions. The metallurgical factors of nodularization and inoculation, and their correlation were also studied. A criterion for modification of cast iron was found, and this criterion was used to examine the complex properties of some magnesium-aluminum compaction agents. From the relation between nodularity and the electromotive force generated by an oxygen probe, a new technique for the prediction of melt compaction degree was developed.