In this paper, the effect of minor impurities contained in tap water has been evaluated in respect of plasma electrolytic oxidation of aluminium alloys in alkaline silicate electrolyte. It was found that chloride and sulphate anions are the main detrimental impurities in tap water for PEO process. PEO processes were carried out with 2024 and 1230 Al alloys. PEO process in electrolyte based on distilled water was taken as a reference. Process evaluation was performed by analysis of anodic voltage behaviour in a time chart form, coating cross sections (SEM and optically), final coating thickness as well as average microhardness. The comparative analysis showed that presence of either chloride or sulphate anions hinders PEO process, reduces process efficiency i.e. final coating thickness. However, it was found that the decrease in process efficiency was not attributed to the destruction of metal-coating interface. Therefore, an extension of the process for about 25% could compensate the reduced efficiency of PEO in electrolytes based on tap water. Moreover, it was found that presence of carbonate improved the efficiency of the PEO coating formation.