A simple, sensitive and reliable method was developed for the determintion of hydrazine in technological waters of NPPs. A pulverized mixture of oxalic acid and p-dimethylaminobenzal dehyde is added to a sample. As low as 1.5 g.dm–3 hydrazine can be determined. The method is dependent of the sample temperature. The agent is stable, readily soluble in water, and not hygroscopic.