In this work we have studied at an ab initio level the lowest (2)A' potential energy surface (PES) of the HCl2 system. This PES is involved in the H(S-2)+Cl-2(X (1)Sigma(g)(+))--> HCl(X (1)Sigma(g)(+))+Cl(P-2) and Cl(P-2) + HCl(X (1)Sigma(+)) --> HCl(X (1)Sigma(+)) + Cl(P-2) gas phase elementary chemical reactions. The former reaction is an important chemical laser while the second one is the most frequently used prototype of heavy-light-heavy reaction. A large number of points on the 2A' PES have been calculated at the PUMP2/6-311G(3d2 f,3p2d) ab initio level. The ab initio calculations show the existence of two angular transition states with negligible or very small barriers to collinearity. This and other properties of the PES are in agreement with previous studies. An analytical expression based on a many-body expansion has been used to obtain a satisfactory fit of the 740 ab initio points calculated, with a root-mean-square deviation within the range of the estimated ab initio method error margin. This analytical representation of the (2)A' PES has been used to evaluate the variational transition state theory thermal rate constants of the above-mentioned reactions, including also the Cl + DCl reaction, and quite good agreement has been obtained when comparing with experimental results. The analytical PES obtained in this work is suitable for use in studies on the kinetics and dynamics of the HCl2 system.