In semiconductors, a widening of the optical band gap occurs because the lower states in the conduction band are blocked. At the same time band gap narrowing also occurs due to many body interactions on doping. This paper reports the analysis of widening and narrowing of optical band gap in sol-gel derived ZnO films moderately doped with Yttrium and heavily doped sputtered ZnO films with aluminum and scandium. The band gap was evaluated using optical transmission data. Carrier concentration was known from the Hall measurements. At high concentrations the effective change in band gap is found to be the difference of the band gap widening and band gap narrowing. At low concentration of dopant the many body theories do not apply and the experiments also show that the band gap narrowing is practically negligible at these concentrations and the effective band gap widening is determined by the band gap widening alone. The chemical nature of the dopant played practically no role. (c) 2006 Elsevier Ltd. All rights reserved.