Single crystal X-ray structures of diag and lat-(η5-C5H4Me)Re(CO)2Br2 have been determined. The diag form crystallizes in the triclinic space group P¯1, a = 6.751(2), b = 8.537(1), c = 9.758(1) Å; α = 96.70(1), β = 93.15(2), γ = 104.96(2)°, V = 534.8(2) Å3, Z = 2. The lat form is monoclinic, P21/c, a = 11.820(1), b = 7.133(1), c = 12.924(1) Å; β = 98.278(8)°, V = 1078.2(2) Å3, Z = 4. The unit cell volumes (per Z) of the two isomers differ by ca 1%. Molecular modelling reveals an energy difference between the two isomers of 1 kJ mol−1 or less. The XRD powder diffraction patterns for the lat isomer produced by crystallization from solution or prepared by thermal isomerization are identical. The evidence thus suggests that the solid state isomerization reaction is a novel example of a reaction which yields a molecular structure determined by crystal packing forces.