Single-crystal X-ray diffraction data have been collected for [Fe₂Os(CO)₁₂] at 120, 223, 288, 292 and 323 K. The two studies at ambient temperature (288 and 292 K) reveal a ≈12 : 1 disorder of the metal triangle as previously reported. At the two lowest temperatures there is no evidence of disorder, while at 323 K the ratio of the major : minor component of disorder decreases significantly to ≈1.4 : 1. Data collected on the same crystal specimens indicate unequivocally that this disorder is dynamic in nature. Two-dimensional exchange and one-dimensional variable-temperature ¹³C magic angle spinning (MAS) NMR spectroscopy showed that carbonyl exchange is rapid above 306 K in the crystalline solid. Two independent exchange processes of similar energy are observed. The first is consistent with the crystallographic evidence, and involves an ‘in-plane’ rotation of the Fe₂Os triangle in steps of 60° within a relatively rigid icosahedral carbonyl manifold. The second involves localised axial–equatorial exchange in the Os(CO)₄ group. The Os L_III and Fe K edge X-ray absorption fine structure spectra are consistent with identical structures being present in tetrahydrofuran solution and in the solid phase.