The elimination of H-2 from C2H5+ was investigated at MP2-FC/6-311+G(d,p), QCISD/6-311+G(d,p), and QCISD(T)/6-311++G(3df,2p)//QCISD/6-311+G(d,p) levels. This elimination is an endoergic reaction that proceeds through a late transition state in which two C-H bonds are synchronously stretched with an energy barrier close to its endoergicity, in agreement with experimental results. Back-donation from the HOMO of C2H3+ to the LUMO of H-2 plays a crucial role in this process. This back-donation depends on the elongation of the C-C bond so that H-2 is eliminated when the synchronous stretching of the two C-H bonds is out of phase with respect to the stretching of the C-C bond.