The purpose of this research was to determine and compare the composition of white mineral trioxide aggregate and gray mineral trioxide aggregate. Electron probe microanalysis results indicated that lime (CaO), silica (SiO₂), and bismuth oxide (Bi₂O₃) were the dominant compounds in each case and were present at comparable levels in either of the types of mineral trioxide aggregate analyzed. It was concluded that the most significant differences observed were between the measured concentrations of Al₂O₃ (+122%), MgO (+130%), and especially FeO (+1000%) when gray mineral trioxide aggregate was compared with white mineral trioxide aggregate.