The donor-free mesityllithium was prepared from the reaction of MesBr with n-BuLi in diethyl ether at −78 °C. The solid-state structure of unsupported mesityllithium consists of C₂Li₂-rings composed of two LiMes units, which interact with adjacent dimers [LiMes]₂, forming a polymeric infinite chain along the crystallographic c-axis (monoclinic space group, P2₁/n). The structure of donor-free mesityllithium reveals short contacts between the C atoms of the mesityl rings and the lithium atoms of neighbouring [LiMes]₂ units. The structure determination of LiMes was performed by X-ray powder diffraction. In addition we have investigated the reaction of LiMes with Me₃SnCl and P₄ for our understanding of the reactivity of donor-free mesityllithium. The heterogeneous reaction of donor-free mesityllithium with Me₃SnCl produces conveniently the stannylated mesitylene Me₃SnMes (triclinic, space group P). White phosphorus reacts with three equivalents of unsupported mesityllithium in benzene to give Li₃P₄Mes₃. In this context it should be noted that a tetraphosphide with an identical LiP-core as in Li₃P₄Mes₃ had been formed in the 1 : 3 reaction of P₄ with the silanide Li[SitBu₃]. The tetraphosphide Li₃P₄Mes₃ was analyzed using X-ray crystallography (monoclinic, space group C2/c).