Formation of a vitreous molecular alloy was observed when a mixture of deoxycholic acid (DCA) and tri-O-methyl-beta -cyclodextrin (TMCD) crystals was subjected to mechanical milling at room temperature. Only a single glass transition temperature T-g, varying with the composition, was observed by DSC. This means that the vitreous state exhibits a single relaxation process as a whole by forming a molecular alloy. The T-g showed a maximum value at the equimolar composition, indicating a strong interaction between the two components. A study of the phase diagram clarified the existence of intermolecular compounds between them. Thermal conductivity of the milled solid of equimolar mixture exhibited a temperature dependence characteristic of glassy materials, p-Terphenyl and tris(hydroxymethyl)aminomethane crystals could not be vitrified. When each of them was milled with DCA or TMCD, vitreous molecular alloys were obtained in a limited composition range. These alloys exhibited also a single T-g and underwent a phase separation on devitrification. Formation of molecular alloys was discussed based on the nature of disorder of the system.