The effects of corrosion of reinforcement in a reinforced concrete structure are manifested in three ways: (1) on the concrete, through cracking and spalling, (2) on the steel, through loss of strength and ductility, and (3) on the reinforced concrete system, through reduced adherence and anchorage of the bars to the concrete. There are numerous studies on the influence of corrosion on the mechanical properties of steel, but most of them are carried out by subjecting the bars to very high degrees of corrosion, often exceeding 20% loss of cross-section, a situation which rarely occurs in practice. In addition, most of the works study the problem by subjecting the bar to a corrosion process along its entire length. However, in many cases, corrosion occurs in a very localised area of the bar. In this work, the mechanical behaviour of rebars subjected to very low levels of corrosion has been studied in two different situations: with their entire length corroded and with localised corrosion in a small portion of the bar. The results show that while the properties related to strength are practically not altered, those related to deformation experience very important decreases even with very low levels of corrosion, being more important when the area where the corrosion occurs is located in a small portion of the bar.