This paper presents results of failure analysis study to characterize the corrosion damage that occurred in the water supply system in Krakow. This analysis includes: electrochemical noise, linear polarization and resistometric measurements. Water aggressiveness of four water intakes was defined using Langelier and Ryznar indices. Results from this indices did not reveal the causes of considerable corrosion losses in water systems. The corrosion rate measurements revealed that water from the one of water intakes is characterized by considerable corrosive aggression. In all studied water subsystems formation of protective layers limiting the corrosion rate was found. Overall analysis reveal the need to implement a on-line corrosion rate monitoring in the water supply system of the city of Krakow.