Com. graphene oxide (GO) nanoparticles were tested for stability in 4 various corm test mediums commonly used in ecotoxicol. studies. The GO concns. were detd. after 1-72 h by spectrophotometry (absorbance lambda = 226 nm). The GO was the most stable in a very soft medium (pH 8.14, ionic power 0.86 mM/L, cond. 43.55 mu S/cm, total hardness (CaCO3) 11.2 mg/L) and the least stable in the hardest medium (7.94, 10.84 mM/L, 671.5 mu S/cm, 250.3 mg/L, resp.).