The control of the composition of mixed organic-inorganic hybrid perovskites by solid-state alloying is a very efficient tool to tune the band gap of the material, leading to enhanced optical performances. At this end, we have elaborated a series of mixed-halide two-dimensional hybrid perovskites (C6H11NH3)(2)[PbBr4-xIx], with 0 <= x <= 4, for which we studied the composition-dependence of the structural parameters and their effect on the white-light luminescence properties at room temperature. The structural analysis revealed the existence of a composition-induced structural phase transition occurring in the range 2 < x < 2.4, associated to a change between Cmc2(1) and Pbca space groups. The optical properties, investigated using optical absorption and photoluminescence measurements, have shown that bromine (Br) to iodine (I) halogen ion substitution redshifts the excitonic optical absorption band, as a result of the hybridization of the valence band structure, built from np orbitals of Br and/or I halogens and 6s orbitals of lead. When iodine was predominant (x > 2), the photoluminescence spectra showed a sharp peak with a relatively small Stokes shift (less than 0.2 eV) attributed to free or bound excitons emissions. In contrast, when the bromide is majority (x < 2), the photoluminescence response consists in a broadband white-light emission with a very large Stokes shift (between 1.2 eV and 0.3 eV), attributed to self-trapped excitons activated by a strong structural distortion of the inorganic sheets. Confronting optical and structural data highlighted the effect of the structure in monitoring the optical properties, since the intensity of the white-light emission increases with the bromine content and was found to excellently correlate with the change of the angular distortion of inorganic octahedra PbX6 (X = I/Br)