Microstructural development of thin-film barium strontium titanate (BaxSr1-xTiO3) as a function of strontium concentration and thermal treatment were studied, using transmission electron microscopy (TEM) and X-ray diffractometry (XRD), Thin films, similar to 250 nm thick, were spin-coated onto Pt/Ti/ SiO2/Si substrates, using methoxypropoxide alkoxide precursors, and crystallized by heat-treating at 700 degreesC. All films had the cubic perovskite structure, and their lattice parameters varied linearly with strontium content. Films with higher strontium concentrations had a larger average grain size, In situ TEM heating experiments, combined with differential thermal analysis/thermogravimetric analysis results, suggest that the gel films crystallize as an intermediate carbonate phase, BaxSr1-xTiO2CO3 (with a solid solution range from x 1 to x = 0). Before decomposition at 600 degreesC, this carbonate phase inhibits the formation of the desired perovskite phase.