A synthetic protocol was developed for the preparation of thin mesoporous TiO2 films with enhanced thermal stability. This objective was achieved by a modification of the procedure of supramolecular templating by a Pluronic P-123 copolymer by an addition of a small amount of phosphoric acid to the precursor solution. Thin mesoporous TiO2 films up to 2.3 mu m thick were prepared via layer-by-layer deposition. The roughness factor exceeding 1300 was achieved on these films. After annealing at 540 degrees C, well-developed 5-6 nm anatase nanocrystals were present in the pore walls of a still perfect mesoporous structure. The pore coalescence and structure collapse during heat-treatment were effectively hindered by the presence of phosphorus. Our P-modified mesoporous TiO2 films were sensitized with the N-945 dye and used as photoanodes of the dye-sensitized solar cells. They reached the conversion efficiency of 5.03 or 5.05% for the film thickness of 1.8 or 2.3 mu m, respectively. Whereas the roughness factor scaled linearly with the number of layers, the solar conversion efficiency reached a constant value for films consisting of eight or more layers. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3250958] All rights reserved.