The water is currently monitored with regard to the acid rain problem. Important indicator components in monitoring rain water are: total acidity, ammonium, nitrate, potassium, calcium, magnesium, sodium, chloride and sulphate. Beside the analytical difficulties caused by the susceptibility to contamination with gases from the laboratory air, the major difficulties in certification are caused by stability problems of the samples. The project was mainly focused on stability studies and 2 certified reference materials (CRM) with low and high element contents, namely 408 and 409, were prepared.