We have investigated the structure of tert-Butanol-Toluene liquid mixtures when confined in the cylindrical channels of MCM-41 mesoporous silicate, and we can conclude on the existence of a micro-phase-separated tubular structure of these binary liquids although fully miscible in bulk conditions. Neutron diffraction experiments with selective isotopic compositions have been performed to vary systematically the scattering length density and thus to highlight different mixture components. The observed apparently anomalous variation of the intensity of the Bragg peaks of the MCM-41 after filling demonstrates without further analysis that the liquid mixture must be inhomogeneous at the nanoscale. Our experimental observations can be rationalized when comparing with the predictions of simple core-shell models. Within this core-shell model we deduce that tert-Butanol segregates as a pore surface layer of about one atomic size surrounding a Toluene-rich core of 2 – 3 atomic layers.