Ba 0.5 Sr 0.5 TiO 3 thin films have been deposited by sol-gel spin coating technique and effect of pH of the precursor sol on the electrical and optical properties has been investigated. Thin films of about 480nm thickness were deposited on platinised silicon (Pt/TiN/SiO 2 /Si) and fused quartz substrates using precursor sols of pH 2.8 and 4.8. The pH of the precursor sol is found to have a pronounced effect on the surface morphology, electrical and optical properties of the deposited BST thin films. X-ray diffractograms and atomic force micrographs show increase in the grain size with the increase in pH of the precursor sol. The grain size was found to increase from about 80nm to about 120nm. The dielectric constant was also found to increase from 325 to 340 with increase in the pH of the precursor sol. The dielectric loss, however, changes slightly from 0.036 to 0.052. As a consequence the leakage current and interfacial charge densities also increase. The optical studies reveal that the band-gap decreases from 3.75 to 3.66eV with increase in the pH. © 2004 Elsevier B.V. All rights reserved.