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Hydrazine (N2H4) and monomethylhydrazine (MMH) are used as propellants in several space-based applications, in which exposure limits as low as 2 ppb have been proposed. This paper reviews the development of hand-held, ambient-temperature instruments that use ion mobility spectrometry (IMS) in the detection of hydrazine and MMH. An instrument, based on early designs, detected hydrazine at 6 ppb with no interference from vapors except for ammonia, but exhibited slow response and recovery times. Performance of a hand-held IMS instrument that used water-reagent ion chemistry was unacceptable. An alternative, using acetone as the dopant reagent, also proved unacceptable, because ammonia-acetone clusters produced substantial interference in the detection of MMH. The goal of the present development effort was to eliminate ammonia interference through altering the ionization chemistry.