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Far-infrared reflection spectra were determined, for the first time, for single crystals of guanidinium aluminium sulphate hexahydrate (GASH) and its solid solution for 5 mol.% chromium isomorph (GCSH). The infrared active transversal and associated longitudinal phonon frequency and the dielectric permittivity functions for E//C and E perpendicular to C have been determined using both Kramers-Kronig analysis and a fitting procedure based on a four-parameter model. The indices of refraction were also measured using the immersion method with Cargille liquid and the results were compared with data obtained by numerical analysis of reflectivity measurements. The optical transmission of the samples were measured in the visible range and three wide energy bands were observed at about 2.6 eV, 3 eV and 4.9 eV for GASH doped with Cr, below the conduction energy zone.