Hydrogen peroxide (H2O2) plays an important role in natural water samples. In this study H2O2 concentrations were quantified in different water samples by chemiluminescence detection. This method was chosen because of its high sensitivity and suitability for determining low concentrations of H2O2. H2O2 is introduced to the oxidation reaction of alkaline luminol solutions in the presence of Co2+ ion catalyst. When these components are mixed, blue light (lambda = 440 nm) is emitted. Maximum chemiluminescence intensity occurs within 2 s after mixing and is continuous for up to a few minutes, permitting accurate measurements at selected delay times. H2O2 concentrations being very low in water samples made us use the standard addition method. Thus the sample luminescences were measured by the addition of 25 muL 0.075 M H2O2 standard solutions. The H2O2 content of water samples was between 0.13 mM (in snow water) and 1.51 mM (in Golbasi Lake water). The results indicated that H2O2 concentrations in surface waters are representative of aquatic life.