We investigated the spontaneous capillaritydriven filling of nanofluidic channels with a thickness of 6 and 16 nm using mixtures of ethanol and water of variable composition. To improve the visibility of the fluid, we embedded metal mirrors into the top and bottom walls of the channels that act as a Fabry-Pérot interferometer. The motion of propagating liquid-air menisci was monitored for various concentrations in transmission with an optical microscope. In spite of the visible effects of surface roughness and different affinity of water and ethanol to the channel walls, the dynamics followed the classical t 1/2 -dependence according to Lucas and Washburn. While the prefactor of this algebraic relation falls short of the expectations based on bulk properties by 10-30%, the relative variation between mixtures of different composition follows the expectations based on the bulk surface tension and viscosity, implying that-despite the small width of the channels and the large surface-to-volume ratio-specific adsorption or chemical selectivity effects are not relevant. We briefly discuss the impact of surface roughness on our experimental results.