The inhibitory effect of the condensation products of thiosemicarbazide (TSC) with furfruldehyde (FrA). crotonaldehyde (CrA) or methyl vinyl ketone (MVK) was investigated as corrosion inhibitors for aluminium and zinc in 0.5 M NaOH using gasonietry and d.c. polarization technique. Correlation of the structural changes with the protection efficiency of CrTSCN, MVTSCN and FrTSCN with the parent TSC wah reported. The experimental results indicate that the presence of adsorbed inhibitors on aluminium or zinc surfaces does not affect the mechanism of hydrogen evolution. Furthermore, polarization curves show that the compounds act as mixed-type inhibitors. Variation of the protection efficiency is evident for the different compounds, with increase of E-electron density of the center of adsorption, by addition of double bonds or by heterocyclic ring, contributing to improvement in protection efficiency. All data were compared and fitted to kinetic-thermodynamic model. The number of active sites on the metal occupied by a molecule ofthe inhibitor and the binding constant ofthe inhibitor to the active site were calculated lor the investigated compounds. The results are compared with fits obtained from the application of other adsorption isotherms.