In this paper, we report and analyze the large blue shift in the optical band-gap of sol-gel derived Ba0.5Sr0.5TiO3 (BST) thin films. BST films of different thickness (150 nm, 320 nm and 480 nm respectively) were deposited layer by layer onto fused quartz substrates by a spin coating technique. The drying temperature for individual layers (pre-sintering temperature) was varied as 400, 500 and 600 {ring operator}C. A large blue shift in the band-gap was observed (with a value 4.70 eV compared to the bulk value of 3.60 eV) for films pre-sintered at 400 {ring operator}C, which decreased with increase in the pre-sintering temperature. To date such blue shifts have been attributed to grain size reduction, stress and the amorphous nature of the films. Here, the blue shift has been correlated with the presence of charge carriers generated by oxygen vacancies and explained on the basis of the Burstein-Moss effect. © 2006 Elsevier Ltd. All rights reserved.