a previous work, experimental evidence Tor different conformations of the weakly bonded CO:CO2 complex has been given in argon matrices, from the analysis of FTIR absorption spectra. A spontaneous interconversion occurs between a high frequency form (HF A and B lines : The CO frequency is shifted toward high frequencies upon complexation) and a low frequency form (LF line : The CO frequency is shifted toward low frequencies upon complexation) which has been characterized at different temperatures between 5 and 12, K. Above 12 K, the HF doubler only remains, Is an attempt to explain tile observed phenomena, concentration, matrix, and isotopic effects are studied, as well as the reverse conversion which is tell times faster than the direct one. A thermodynamical analysis of the conversion is performed. From the temperature behavior of the equilibrium constant ii, we can suppose that the conversion exhibits two regimes : 4 low temperature one below 8 K and a high temperature : one between 8 and 12 K. A reasonable explanation could be that three conformations are involved : A more stable one corresponding to the narrow low frequency line (probably tile T shape observed in molecular beam experiments) and two closely related less stable conformations which give rise to the A and B lines of the high frequency doublet. The energy differences between these conformations are small : Delta H = -140 J mol(-1) and -337 J mol-l between the LF form and the HF B and it forms, respectively. A subtle mechanism, involving a 3 concerted exchange between the CO molecule of the complex and a nearest argon atom, is proposed to explain the experimental observations.