Decamethylmanganocene (Cp2*Mn) was investigated by x-ray methods at 240, 225, and 125 K, to study a reversible phase transition between 255 and 238 K (DSC thermogram). At 240 K the same space group C2/c is obsd., whereas after the phase transition at 225 and 125 K the space group Cmca is found. In both phases the Cp* rings are staggered and slightly slipped, but less at 225 and 125 K compared with 240 K. The phase transition is traced back to a change of the mol. packing in the crystal lattice: with decreasing temp. the mols. cannot be packed closer in the monoclinic phase (C2/c), because the intermol. H...H distances already remained under the sum of the van der Waals radii. In the orthorhombic phase (Cmca) (elongation of the a-axis and contraction of the b-axis) the repulsion along the b-axis is avoided.