AbstractSome gas phase processes during the vacuum vertical directed crystallization of α‐Al218O3are studied in this paper. An effect of these processes on the H, C, N concentrations in the crystals is discussed. The detected pulsations of partial H2and C18O pressures are a result of bubble destruction at the melt‐gas interface. The C18O bubbles are formed because of chemical interaction between the melt and carbon‐containing phases of container material. Besides the container wall, the H2bubbles can be generated also at the crystallization front supersaturated by H2. The perfect α‐Al2O3and α‐Al218O3crystals grown under the same conditions have noticeable differences of their density, microhardness and lattice parameters. This is explained by an isotopic substit