Proton transfer is a very common and important chemical step in many systems. Despite its apparent simplicity, a correct description of this chemical process is difficult from a theoretical point of view. It requires a correct and simultaneous description of a bond breaking and a bond formation. The situation is even much more complicated when two protons are implied. This is the case for monothiooxalic acid, for which two different types (1,3- and 1,4-prototropy) of proton transfers can be invoked. A further problem is the type of the reaction (concerted or not). This paper reports a complete investigation of the potential energy surfaces: characterization of equilibrium points and transitions states. The main conclusion is: the 1,4-prototropic mechanism, mainly considered as a one step concerted exchange of protons, is the most favored from an energetic point of view.