An effect of factors determining water quality in the water supply network in Kraków is assessed. Data collected over a four-year research period included quality parameters of water taken from the water distribution system in the period between 1 January 2011 and 31 December 2014. In analysis the supply zones of four municipal water treatment plants in Krakow were considered. The selection of 29 water sampling points within the supply area allowed water quality to to be compared with respect to operational and technological aspects. Factor analysis enabled 4 components explaining correlations between tap water quality variables to be distinguished. It follows from the research performed that the obtained factors applied to 77% of overall water variability. The highest share was assigned to factor 1 that explained in 32% the chemical composition of water under consideration and was correlated with calcium, conductance, nitrates (V), magnesium and to a moderate extent with ∑ THM (with negative sign).