This study takes place in the Central region of Cameroon, in the humid tropics with a mean annual rainfall of 1600 mm. A systematic sampling of rain was conducted during rainy periods (from February to November) of 2007 and 2008. The isotopic content of precipitation collected in Yaounde show wide variation (180 = -4.30 ‰ and 1.96 ‰ and 2H = -26.63 and 27.91 ‰) due to the climatic conditions, rainfall events and the origin of air masses generating precipitation. This isotopic variation is directly linked to different factors which rainfall is subject to: evaporation during fall and mass effect. Chemically, the rainfall is calcium bicarbonate and has a low mineralization, characteristic of unpolluted continental environments. The origin of the ions in the rains are diverse: marine (Na+, Cl-); terrigenous (Ca2+, Mg2+, HCO3-) and anthropogenic (NO3-, SO42-, K+, Cl-).