The influence of some Pyridoprymidine derivatives on the corrosion of carbon steel in 0.5 M HCl was investigated using weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM) techniques. The inhibition efficiency increases with increasing the inhibitor concentration, but decreases with increasing the temperature. The inhibitors were adsorbed on the carbon steel surface following Temkin's adsorption isotherm. The electrochemical results indicated that all the investigated compounds act as mixed-type inhibitors. Some thermodynamic parameters for corrosion and adsorption processes were determined and discussed. The mechanism of inhibition process was discussed in the light of the chemical structure and quantum-chemical calculations of the investigated inhibitors.