Three new benzaldehyde thiosemicarbazone derivatives namely benzaldehyde thiosemicarbazone (BST), 4-carboxyl benzaldehyde thiosemicarbazone (PBST) and 2-carboxyl benzaldehyde thiosemicarbazone (OCT) were synthesized and their inhibition effects on mild steel corrosion in 0.5 M H 2 SO 4 solution were studied systematically using gravimetric and electrochemical measurements. Weight loss results revealed that PBST exhibited the highest inhibition efficiency of 96.6% among the investigated compounds when the concentration was 300 µM. The analysis of polarization curves indicated that the three benzaldehyde thiosemicarbazone derivatives acted as mixed type inhibitors and PBST and OCT predominantly anodic. The adsorption process of all these benzaldehyde thiosemicarbazone derivatives on Q235 steel surface in 0.5 M H 2 SO 4 solution conformed to Langmuir adsorption isotherm. Scanning electron microscopy was conducted to show the presence of benzaldehyde thiosemicarbazone derivatives on Q235 mild steel surface. The results of theoretical calculations were in good agreement with that of experimental measurements.