Corrosion inhibitors recommended for subsequent application onto existing structures are a kind of repair material which is cost-efficient and easy to apply. According to the manufacturers, these substances penetrate very rapidly, however, the mechanisms which are responsible for fast migration and involved in transport (capillary suction, diffusion in dissolved state and diffusion in the gaseous phase) have not been sufficiently investigated so far. The major part of the study therefore focuses on the transport of the corrosion inhibitor SIKA Ferro-Gard (FG 903) in cement paste and concrete. It contains two active ingredients, namely a phosphorous compound and an aminoalcohol. It has been shown that the phosphorous compound forms an insoluble salt with the calcium ions of the cement and, therefore, does not penetrate from outside into the concrete and cannot develop an inhibiting effect there. The aminoalcohol, on the other hand, remains largely dissolved in the pore liquid, thus providing optimal conditions for high mobility. The analysis of the transport mechanisms involved has revealed that diffusion in dissolved state is by far the most efficient transport mechanism. While basically transport of the aminoalcohol via the gaseous phase is possible, it plays an inferior role only. Surprisingly, the substance is hardly absorbed by concrete by capillary suction, but remains close to the concrete surface at first.