Stable isotopic composition and radiometric age were measured for groundwaters in the Emilia area of the Po sedimentary basin (Northern Italy). Waters from unconfined shallower aquifers, deep circulating continental waters and sodium chloride waters (relic waters) squeezed out from underlying marine formations during past geological events were considered. Waters from supposed confined aquifers (paleowaters) are usually depleted in 0–18 (δ¹⁸O < -10‰) and their δ¹⁸O–δD relationships are not consistent with their belonging to the present hydrological cycle. Isotope data on sodium chloride waters suggest mixing of relic saline waters with deep circulating continental waters. Processes affecting stable isotope composition of groundwaters in unconsolidated sediments are discussed. Oxygen isotope chemistry is considered to be a rather conservative parameter at least at low temperatures, thus able to give paleoclimatic information. Some ¹⁴C age data on deep circulating groundwaters suggest permanence times within the aquifers greater than 15, 000yrs in some cases. These data agree with paleoclimatological deductions in pointing out the presence of paleowaters in the Po sedimentary basin.