In this communication, we present a simple, completely reversible and well performing optical microsensor for the quantitative determination in liquid hydro‐alcoholic binary mixtures, based on the simple and low cost porous silicon (PSi) nanotechnology. In our sensor device, the transducer element is a PSi optical microcavity (PSMC) which is constituted by a Fabry‐Pérot layer between two distributed Bragg reflectors. When the device is exposed to the mixture, the partial substitution of air in the pores changes the average refractive index of PSMC, causing a red shift of its reflectivity spectrum, characteristic of the concentration of each component. We have characterized, in the 0‐100% composition range, three hydro‐alcoholic binary mixtures (water‐Ethanol, water‐ Methanol and water‐ Isopropanol) which exhibit very different behaviour. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)