Single-crystal X-ray structures have been determined at intermediate stages in the single-crystal to single-crystal trans-to-cis thermal isomerization of (eta(5)-C5H4Me)Re(CO)[P(OPh)(3)]Br-2 at 150 degreesC. Unit cell parameters and site occupancy factors linked to the induced disorder were monitored and found to follow first-order kinetics in the initial (fast component) stage of the reaction. A rate constant of 30 x 10(-6) s(-1) (0.11 h(-1)) was obtained to first-order approximation for the reaction. The irreversibility of the isomerization from single crystals of the cis form was also confirmed. Fourier and difference Fourier maps suggest a novel 2-fold rotation mechanism for the trans-to-cis isomerization reaction. This mechanism entails a unidirectional anticlockwise movement of the CO ligand and only one Br atom.