The reducing characteristics of blast furnace slags (BFSs) and their influence on the redox potential E(h) of slag-cement pore solutions are now well known, but beyond certain specialized areas of blended-cement applications this property has received little attention. The continued and increasing use of slag cements in the construction industry will, however, require consideration of the effect of the electrochemically reducing pore solution on the passivation of steel reinforcement in the cement system. A theoretical interpretation of preliminary results derived from electrochemical and electron microscope studies on steel embedded in Portland cement (OPC) and slag cement (BFS-OPC) matrices is presented. The model proposes that the reducing components of BFS will impede the early formation of oxidized passive layers on steel in high BFS-loaded BFS-OPC concretes, and may gradually degrade previously formed oxide layers. Thus, the corrosion behaviour of steel in BFS-OPC matrices is considered to be different from that in other cementitious systems.