Magnesium alloy AZ91 contains 0.1 to 0.3 mass% manganese as an alloying element. Such manganese always reacts with aluminum and produces various compounds. Actually, numerous Mn-bearing particles are visible in polished surfaces under an optical microscope. Some Mn-bearing particles are also expected to be present in molten alloy. We believe that some Mn–Al compounds are closely related to the superheating effect. However, it is not clear specifically what compounds exist in molten alloy and how contribute to grain refinement by superheating. In order to study this subject, we utilized a diffusionless process based on rapid solidification. Two grams of AZ91 alloy was melted in a stainless steel tube and then injected onto a copper wheel rotating at high-speed under various conditions to obtain cast ribbons. Cast ribbon was analyzed by an X-ray diffractometer, an electron probe micro-analyzer and a transmission electron microscope. These analyses indicated that the cast ribbons consist of single phase and that the structure is quite homogeneous, i.e., diffusionless solidification occurs due to rapid cooling. Manganese-bearing particles completely disappear in the melt around 963 K and superheat temperatures, which is inconsistent with presently accepted superheat mechanisms, the temperature-solubility nucleation theory and the temperature-phase relationship theory. In the meantime, cross-shaped Mn-bearing particles are often observed in the ribbons when the melt is cooled from superheat temperatures to the injecting temperature (973 K) before injecting. This treatment may facilitate the formation of these particles.