Thickness determination of Ta2O5 thin films, deposited on the glass substrates and metallic indium and gold thin films on both glass and aluminum substrates, were performed by neutron activation analysis. Thickness determination of these thin films were made by comparing gamma -rays emitted from the radio-isotopes in the thin film with the substrate material followed by the neutron irradiations. The method led to determination of the film thicknesses without using any standard sample. A complementary optical transmission measurement was also applied on multi-layered Ta2O5 thin films for determining the individual layer densities. (C) 2001 Elsevier Science Ltd. All rights reserved.