The inhibition of steel corrosion by thiourea and cations such as aluminum, calcium, and magnesium under cathodic control in both 3. 5% NaCl solution and seawater was investigated. When the cathodically controlled potential was less electronegative than minus 1. 00 V, protection in a 3. 5% NaCl solution was incomplete. This incomplete protection was compensated for about 50% by addition of either aluminum ion or thiourea in the solution and compensated for about 90% by the combined use of both at 50 ppm. Based on measurements of pH and galvanic current a possible mechanism is proposed.