Omphacite in ultra-high pressure (UHP) eclogites from the Pohorje Mountains in Slovenia, south-eastern Alps, has been investigated by electron microprobe (EMP), infra-red (IR) and Mössbauer spectroscopy to determine OH concentrations and related incorporation mechanisms. Results from polarized IR measurements reveal high contents of structurally bound OH, varying from 530 to 870 ppm H2O. Characterization of omphacite composition by EMP analysis and Mössbauer spectroscopy shows that all samples contain vacancies at the M2 position, which can be expressed as a Ca-Eskola component (Ca0.5□0.5AlSi2O6). The amount of the Ca-Eskola component displays a positive correlation with the OH concentration, which confirms results from previous studies. The occurrence of precipitated quartz rods in some samples indicates that primary omphacite contained a larger Ca-Eskola component. Extrapolation of the observed trend-line for the Ca-Eskola and OH contents point to an original OH concentration around 1500 ppm H2O for these samples. The high water contents observed in omphacite are considered to be linked to the UHP origin of the eclogite rocks.