The effect of a commercial inhibitor (Chemtec FI30) on the corrosion of P235 carbon steel in water (demineralised and/or tap water) with the addition of a commercial biocide (Chemtec BI01) was studied in the present work. This study was carried out by weight loss and electrochemical methods, while specimen’s surface was examined using optical microscope. Results obtained from gravimetric measurement revealed that P235 carbon steel corroded in aqueous solution, i.e., in a solution of demineralised and tap water (50 : 50) and a biocide (Fig. 1), and in tap water with a biocide (Fig. 5). By analysing the surface of carbon steel with an optical microscope, general and pitting corrosion was observed, which was more pronounced in tap water with biocide (Figs. 2 and 6). The addition of a commercial inhibitor significantly reduced the occurrence of corrosion damage on the surface of carbon steel (Figs. 3, 4, 7, and 8) and decreased the rate of carbon steel corrosion in all aqueous solutions (Table 1). Electrochemical investigations performed by open circuit potential measurement, linear polarisation and potentiodynamic polarisation measurement showed that the inhibitor Chemtec FI03 shifted the potential to more positive values (Figs. 9 and 10), increased polarisation resistance (Table 3), decreased the rate of carbon steel corrosion, and had high efficiency in all tested solutions (Table 4). The results obtained showed that this commercial inhibitor could be used as an effective inhibitor for the corrosion of carbon steel in aqueous media. [ABSTRACT FROM AUTHOR]