The development of microporous ceramic thin layers is of prime interest for sensors or gas separation membranes working at high temperature. Microporous silica membranes can be easily prepared by the sol-gel process. However the microporosity of pure silica is rapidly modified by steam at high temperature. One way to improve hydrothermal stability is to use mixed-oxide membranes. In this work, microporous silica-alumina membranes were prepared by a simple and robust sol-gel method. Tetraethoxysilane was mixed with an acidic alumina hydrosol. Urea was added for preparing the alumina hydrosol, for controlling the mixed-oxide network polycondensation and also as porogen agent. FTIR and Al-27 NMR spectroscopic analyses showed that for Si/Al molar ratios up to 6/1, homogeneous mixed oxides were obtained with a random distribution of Al and Si atoms in the oxide lattice based on tetrahedral units. The derived supported layers were crack-free as demonstrated by scanning electron microscopy (SEM) observations. Their microporosity was investigated using ellipsoporosimetry (EP) with films supported on flat dense substrates. He, N-2 and CO2 permeance measurements were performed for membranes deposited on porous tubular substrates. The measured values of He/N-2 and He/CO2 ideal selectivities are in agreement with the microporous nature of the prepared layers.