The results of the spectroscopic and electrochemical study of SO2 solutions and of polythionite solutions (i.e. solutions where SO2 is reduced chemically) in HMPA, allowed us to build a model describing the reduction of SO2 in non-aqueous solvents. Simulations and adjustments of this model to the experimental data led to the determination of the parameters of the reactions. This mechanism is shown to explain the observations for all solutions, both in semi-infinite diffusion and in thin layer conditions, and it also describes the composition of any polythionite solution. The model also explains the reduction of SO2 in other non-aqueous solvents. The analysis of the mechanism shows that the stability and electroactivity of the species involved can be related to their structure.