A gas chromatography - mass spectrometric method for determination of unsymdimethylhydrazine (UDMH) in soil and water has been developed. The method is based on derivatization of UDMH with 2-nitrobenzaldehyde, 4-nitrobenzaldehyde 4-chlorobenzaldeyhde, 4-cyanobenzaldehyde 4-dimethyaminobenzaldehyde, benzaldehyde and furfuraldehyde, followed by analysis of the derivative using gas chromatography - mass spectrometry. The effect of ambient air and antioxidants (pyrogallol and TiCl3) to absorb dissolved oxygen from the reagents has been investigated. The method proves to be useful for UDMH analysis in soil and water, and has been used for environmental site assessment at a former missile base in Ukraine.