The inhibition of the corrosion of Helwan mild steel in hydrochloric acid solutions (1.0 M) by carbamide, urea, and thiocarbamide derivatives, thiourea and thiosemicarbazide, at relatively elevated temperatures were investigated. The inhibition efficiency of each inhibitor was studied extensively by weight-loss and polarization measurements. The corrosion rate was found to depend on solution temperature and inhibitor type and concentration. In case of urea, the variation of R(corr) with temperature is more remarkable than that of inhibitor concentration. Further, the rate of corrosion at temperatures < 85 degrees C decreases with the increasing of inhibitor concentration and the trend is reversed at higher temperatures. On the other hand, the inhibition efficiency at all temperatures increases with the concentration of thiourea up to a critical concentration above which it decreases, the extent of which depends on inhibitor concentration and solution temperature. The inhibition action of the inhibitor used follows the order: Thiourea > Thiosemicarbazide > Urea A quantitative correlation between inhibitor structure and inhibition efficiency and its temperature dependence was explained. The mechanism and mode of inhibition of the tested inhibitors was proposed.