The isotopic and chemical studies were carried out in three regions of the Indus Basin to ascertain the source of salinity. Samples collected from the Faisalabad are in Rechna Doab, Chaj Doab and Mardan Valley, were analysed on mass spectrometer for D/H, 18O/16O and 34S/32S ratios. Electrical conductivities and pH measurements were made in situ while the analyses of various cations/anions were made in the laboratory. The isotopic results show that the salinity is mainly due to the dissolution of sediments salts by the infiltrating sweet water. The chemical data also support this conclusion. The rise in salinity is also partly due to the use of fertilizers and evaporation processes. (author).