The optical properties of single crystals of the topological insulator Bi1.1Sb0.9Te2S are studied by optical spectroscopic ellipsometry using Beattie method in the range of 0.5 - 5 eV. The real and imaginary parts of the complex dielectric permittivity are obtained. It was found 8 critical points based on the analysis of the second derivative of complex dielectric permittivity. These critical points correspond to interband transitions from valence band to conduction band and have different dimensionality. Also, it was found that plasmon behaviour for Bi1.1Sb0.9Te2S emerges for photon energies higher than 1.86 eV.