Single crystals of EuO have been grown from melts containing excess Eu-metal. i.r. absorption and conductivity measurements were made on these crystals and the results correlated with the crystal growth parameters. It is seen that as the concentration of europium in the initial melt is increased, the growth temperature is lowered and the crystals become richer in europium. The variations in the i.r. absorption and conductivity were studied and are shown to be strongly related to composition. According to the i.r. and conductivity behavior, the EuO crystals can be classified into five types, with compositions ranging from oxygen-rich, to stoichiometric, to europium-rich. The oxygen-rich EuO crystals are insulators and exhibit i.r. absorption which is attributed to Eu3O4 and to Eu3+. Only weak i.r. absorption is seen in the stoichiometric crystals. Those with a deficiency of oxygen show a new i.r. absorption and have either metallic conduction or show large insulator-metal transitions which are attributed to oxygen vacancies. A new phase diagram has been constructed which can be used as a guide for growing EuO crystals. Several crystal growing methods are also discussed. © 1972.