The inhibition effect of 2-pyrazinecarboxamide (PCA), KI and their synergistic effect on the corrosion of carbon steel in 0.5 M H2SO4 solution was studied by electrochemical techniques. Scanning electron microscopy (SEM) was used to characterize the steel surface. The inhibition efficiency increases with the concentration of PCA and KI and increase further in the presence of PCA/KI mixture. The open circuit potential and polarization measurements suggested that PCA and its mixture with KI act as mixed-type inhibitor; while, KI only acts as mixed-type inhibitor with a predominant anodic inhibitor. The adsorption of PCA and KI on the metal surface in 0.5 M H2SO4 solution obeys the Langmuir adsorption isotherm. The thermodynamic parameters Kads and △G°ads are calculated and discussed. The probable inhibitory mechanism is proposed from the viewpoint of adsorption theory. The synergism parameter suggests that there is a cooperative mechanism between the iodide anion and PCA cations.