(Ba-0.5 Sr-0.5)TiO3 thin films have been prepared by the sol-gel technique. A correlation between the evolution of optical constants and structure shows that the refractive index and optical band gap approach the values corresponding to the bulk stoichiometric compound only at temperatures greater than 600 degrees C. The onset of crystallinity (at 600 degrees C is accompanied by an increase in refractive index and decrease in optical band gap. The dispersion behavior after decomposition at 500 degrees C, when fitted into the single effective oscillator model, indicates the presence of organic residue which causes the amorphous nature as well as large deviation in band gap and lower refractive index. Beyond 600 degrees C the films approach the bulk band gap value of 3.6 eV with a maximum refractive index of 2.05 in the region between 500 and 2500 nm as against the 1.75-1.81 at 500 degrees C. The crystallite sizes are of the order of 20 nm even at temperatures as high as 800 degrees C. These results clearly indicate that complete oxidation and crystallization occurs at temperature >= 600 degrees C. The microware dielectric constant and loss of the films were measured at two spot frequencies of 10.059 and 12.14 GHz and were found to increase with increase in film thickness. At 10.059 GHz the hiahest dielectric constant achieved is 194.2 for 300 nm thick film. (c) 2007 Elsevier B.V. All rights reserved.