A new method for the preparation of fluorescent mesoporous La–Ce phosphate films by spin coating is reported here. The structure is obtained by a pore-confined synthesis involving an in situ growth of La_0.5Ce_0.5PO₄ nanoparticles inside the pores of a mesoporous silica thin film and the subsequent partial sintering. The removal of the sacrificial mesoporous silica matrix is achieved by chemical etching, which leaves on the substrate a La_0.5Ce_0.5PO₄ mesoporous film exhibiting good adhesion properties and high fluorescence intensity that can be quenched by a water solution containing organo-phosphate bearing nitrophenyl groups. The sensing capabilities of these materials have been tested for the detection of pesticides in water.