Accurate ab initio calculations are performed in order to characterize the most stable isomers of empirical formula C₃S⁺ and the role of C₃S⁺ during the ion–molecule reactions between the C₃⁺ triatomic molecular ions and S atoms. The linear form l–C–C–C–S⁺(X²Σ⁺) is found to be the most stable isomer followed by a three-membered carbon ring with an external S {c-C₃S⁺(X²A₁)}. The ion–molecule reactions are investigated by doing suitable 1-D cuts of the 6-D potential energy functions (PEFs) of the lowest electronic states of C₃S⁺. After its formation, the C₃S⁺ intermediate may dissociate leading, in addition to the charge transfer products, to C, C⁺, C₂, C₂⁺, CS, CS⁺, C₂S and C₂S⁺ species. Generally, the dynamics of these reactions are found to involve several electronic states of C₃S⁺ and their mutual couplings, including Renner–Teller couplings, spin–orbit interactions and vibronic interactions. These couplings can take place before and/or after intramolecular isomerisation processes.