The present work studied the formation of new phases by adding alloying additions (Ca, Sr, and Ce) in AZ31 alloy at micro-levels (approximately 0.2 wt.% per element). Quantitative electron probe micro-analysis (EPMA) was used to identify the compositions of these phases; a comparison in the morphology and distribution of an identical phase in different alloy systems was made; and the thermodynamic software of FactSage™ was utilized to calculate the possible intermetallic phases during solidification. The results of EPMA showed the existence of phases other than Mg₁₇(Al, Zn)₁₂ and Al₈Mn₅ in AZ31 alloy, namely: Al₂Ca, Al₂Sr, Al₁₁Ce₃, Al₈CeMn₄, and Al₂(Ca Sr); and these new phases were thermally stable at 450 °C for 10 h. The morphology and distribution of second phases in different alloy systems were self-consistent. These phases were well predicted by FactSage™ with recent light alloy database.