Nine omphacitic clinopyroxenes with a jadeitic content between 25% and 57% from the Udachnaya and Zagadochnaya diamond-bearing kimberlites were investigated using single-crystal X-ray diffraction, electron microprobe analysis, and Mössbauer and infrared spectroscopy. Crystal-structure refinements, incorporating constraints from chemical analyses and Mössbauer data, show a significant concentration of vacancies at the M2 site in three of the samples. The cell volumes of the non-stoichiometric samples plot below the linear trend defined by stoichiometric diopside-jadeite compositions, where the deviation from the linear trend is positively correlated with the concentration of vacancies. Charge-balance appears to be achieved primarily through substitution of Al3+ for Mg2+ on the M1 site, which causes a contraction of the M1 site volume compared to stoichiometric clinopyroxenes with the same composition. Vacancies are not associated with OH− incorporation, as the sample with the highest concentration of vacancies contains only 89 ppm (wt) H2O. Significant hydrogen loss through iron oxidation during ascent is ruled out based on the low Fe3+ concentrations. H content in omphacite samples from kimberlites could provide a sensitive tool for determining fH2O during diamond formation.