A tetraalkylphosphonium salt having at least one branched alkyl chain containing 9 to 100 carbon atoms in total, an organically modified layered silicate containing the tetraalkylphosphonium salt having at least one branched alkyl chain containing 9 to 100 carbon atoms in total between layers of a layered silicate and a composition comprising the organically modified layered silicate and an organic solvent or a thermoplastic resin. There are provided a tetraalkylphosphonium salt for a layered silicate having thermal stability and dispersibility superior to those of conventional organophilized layered silicates, an organically modified layered silicate containing the tetraalkylphosphonium salt and a composition thereof.