This paper reports a study on the deposition of sol particles for the preparation of thin and ultra-thin electrolyte membrane layers (thickness < 5 mu m-50 nm), which cannot be produced with regular powder-based processes. For the deposition process, a range of coating liquids with varying particle sizes, covering the complete range between standard suspensions with a particle size of several 100 nm and nano-particle sols, was prepared. In the first part, it is demonstrated that a colloidal sol route can be used for membrane formation on a regular macroporous SOFC anode (NiO/zirconia), when the sol particle size is adapted to the pore structure of the anode (particle size similar to 200 nm). SEM characterization indicated a thickness in the range 3-4 mu m after calcination at 600 degrees C and ca. 2 mu m after sintering at 1400 degrees C, far below the limit for conventional powder-based deposition methods. In the second part, ultra-thin zirconia and ceria membrane films are prepared by spraying sols containing nanoparticles (average size 5-6 nm). The layers show a thickness of similar to 100 nm, a very narrow particle size distribution and tight ultra-microporous structure, which allows a sintering treatment below 1000 degrees C, and can be used as an additional electrolyte layer for improving the leak rate of the cell or as diffusion barrier. (c) 2008 Elsevier B.V. All rights reserved.