The determination of the chemical composition of rain-fall related to the origin of the air masses, in two urban sites in Greece during the year 2006, is investigated in this study. Two model automatic rain samplers were installed, the first in the city of Larissa, Thessaly, central Greece and the sec-ond in Heraklio, Attica, a northern suburb of Athens. The concentrations (ppm) of the major cations (H + , Na + , K + , Ca 2+ , NH 4 + and Mg 2+) and major anions (NO 3 -, NO 2 -, HCO 3 -, and SO 4 2-), as well as total hardness (ppm CaCO 3), pH and electric conductivity in 25 o C (µS/cm) for 27 rainfall sam-ples -11 samples in Larissa and 16 samples in Heraklio (Athens)-were determined. In Larissa, the figures of pH range from 5.13 to 6.13 while in Heraklio (Athens), the pH within the range 5.62 to 7.88 indicates a shift of the rainfalls towards alkalinity. The electric conductivity in Larissa ranges from 16.30 µS/ cm to 110.60 µS/cm and in Heraklio (Athens) from 7.00 µS/ cm to 151.00 µS/cm. The analysis showed that Ca 2+ and Mg 2+ appear the highest concentrations out of the examined cations, while HCO 3 -and SO 4 2-present the highest concen-trations within the anions. Moreover, in order to find out the origin of the air masses, the air mass back trajectories were calculated using the HYSPLIT 4 model of Air Re-sources Laboratory of NOAA for two different levels: 1500 and 3000 m (a.m.s.l.).