This paper investigates the reactions involved when LiNi_0.8Mn_0.1Co_0.1O₂ (NMC 811), which is one of the most promising positive electrodes for the next generation of lithium-ion batteries, is leached by hydrochloric acid. This study shows that the leaching behaviour of lithium is quite different than those observed for nickel, cobalt and manganese contained in NMC 811 since lithium dissolution is faster than those observed for nickel, cobalt and manganese. Analysis of leaching kinetic data evidenced that NMC 811 dissolution occurs in two steps. In the first step, NMC is transformed into a new phase which contains less lithium (2.8 < n < 3.6): 1^st step: where M = Ni, Mn, Co. In the second step, the new phase is dissolved (limiting step): 2^nd step: . Finally, the overall reaction of NMC 811 leaching by hydrochloric acid can be written as: 2LiMO_2,(s) + 8HCl_(l) ⇌ 2LiCl_(l) + 2MCl_2,(l) + 4H₂O_(l) + Cl_2,(g).