A Soxhlet washing method for the purification of the NHC IMes precursor; [IMesH][Cl], has been devised. Post purification, the washed material gives improved yields of IMes using conventional synthetic protocols. Structural analysis of material obtained from the washings indicates formation of the acetone solvate; [IMesH][Cl].acetone [triclinic space group P: a = 8.3676(17), b = 11.315(2), c = 13.246(3) Å, α = 70.27(3), β = 80.10(3), γ = 76.55(3)°], which loses solvent at ambient temperature, and the stable monohydrate; [IMesH][Cl]·H₂O (monoclinic space group P2₁/c: a = 8.3198(17), b = 15.400(3), c = 15.635(3) Å, β = 99.40(3)°], which forms in low yield upon use of undried acetone during purification. Both solvates exhibit extensive C–H⋯Cl⁻ hydrogen bond binding of the chloride counterion, that of the hydrate accompanied by a strong O–H⋯Cl⁻ hydrogen bond to the lattice water. This presumably contributes to the solvent stability of the hydrate vis-à-vis the acetone solvate.