The thermal solid-state diag-lat isomerisation of (eta(5)-C5H4R)Re(CO)(L)Br-2 (R=Me, tBu and SiMe3; L=CO, CNC6H3Me2, P(OMe)(3) and P(OPh)(3)) has been studied by DSC, and thermomicroscopy. In all instances, where isomerisation of (eta(5)-C5H4R)Re(CO)(L)Br-2 (1) was observed, the diagonal (trans) isomer converted to the lateral (cis) isomer in an exothermic reaction. The effects of heating rate and crystal size on the isomerisation reaction involving (1) (R=Me, L=CO) has been studied. Microscopic studies on 1, (R=Me, L=CO and P(OPh)(3); R=tBu, L=CNC6H3Me2) showed that the solid-state isomerisation reactions proceeded via a nuclear formation and growth mechanism.. Interestingly, no crystal cracking was observed when (1) (R=Me, L=P(OPh)(3)) underwent a thermal solid-state isomerisation reaction.