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. This was achieved using ion chemistry based on adding 5-nonanone as dopant in the ionization region of the IMS. This modification gave product ion drift times for MMH and ammonia of 12 and 14 ms, respectively, with a linear range from 10 to 600 ppb. Slow response and recovery was found to be improved by elevating the membrane and inlet temperatures. Calibration of this instrument performed before and after Space Shuttle mission STS-37 revealed no more than 5% difference between calibration curves, with no notable loss of equipment function.