Activation parameters have been determined for the 2-vinylphosphirane-3-phospholene rearrangement in the cycloheptane annellated series 7 --> 8. For the thermal reaction, the mean activation parameters (Delta H-double dagger = 125 +/- 5 kJ mol(-1) and Delta S-double dagger = 1 +/- 14 J K-1 mol(-1)) are as expected for a biradicaloid character of the transition structure. In the presence of Cu(I) as a catalyst, the activation parameters for this conversion (Delta H-double dagger = -80.2 +/- kl mol(-1) and Delta S-double dagger -100 +/- 7 J K-1 mol(-1)) are strikingly different and more in line with a concerted pathway.