The isomerisation of ttt-RuCl2(RNC)2(PPh3)2 (R=2,6-xylyl, tBu, iPr, benzyl, 2-OMe-4-Clphenyl) to cct-RuCl2(RNC)2(PPh3)2 has been carried out in the solid state. The reaction is first order and kinetic measurements have yielded activation energies of 210 kJ mol−1 (R=tBu) and 221 kJ mol−1 (R=benzyl) for reactions performed between 160 and 180°C. XRD analysis of the solid state reaction of ttt-RuCl2(tBuNC)2(PPh3)2 has revealed that the cct-isomer produced is a polymorph of that produced by recrystallisation of the pure cct-RuCl2(tBuNC)2(PPh3)2 isomer. A possible mechanism for the isomerisation reaction involving rotation of the two small groups (Cl, RNC) is proposed.