The properties of reinforcing steel must comply with the fundamental assumptions underlying structural codes of practice on which verifications of structural behaviour are based. This is of predominant importance not only for design but also for assessment, where demand of deformation capacity of Reinforced Concrete (RC) is often considered implicitly. Yet, the actual deformation capacity directly depends on the mechanical properties and condition of reinforcing steel. Experimental results indicate that corrosion significantly reduces ductility, yield strength and ultimate tensile strength of reinforcement. It is however unclear which extent of corrosion may actually lead to a decrease of ultimate strain of the reinforcing steel below the minimum requirements following from the deformation capacity demands and how to consider such situations in the assessment of structures. The majority of information about the effects of corrosion on the mechanical properties of reinforcing steel is obtained from modern, ductile steel that has been corroded under an accelerated process, rather than under a more realistic long-term exposure. Moreover, experimental studies are fragmentary in scope and therefore often do not consistently consider important aspects such as the type of reinforcement steel and the morphology of the corrosion defect. Consequently, regression models developed to capture the effect of corrosion on reduction of the material properties of reinforcing steel, which are based on the empirical results, are usually characterised by large uncertainties. Given the lack of systematic consideration of the factors governing the reduction of the mechanical properties, such models tend to oversimplify the phenomena and are only to a limited extent suitable for the assessment of the structural safety of corroded structures. In this paper, governing factors for the reduction of mechanical properties of reinforcing steel due to pitting corrosion are investigated by an analytical model, accounting for the morphology of the corrosion defect and essential steel properties. Validation of the model shows good agreement with detailed FEA and experimental data.