The gas-phase reactions of protonated hydrazine (hydrazinium) with organic compounds were studied in a selected ion flow tube-chemical ionization mass spectrometer (SIFT-CIMS) at 0.5 Torr pressure and similar to300 K and with hybrid density functional calculations. Carbonyl and other polar organic compounds react to form adducts, e.g., N2H5+(CH3CH2CHO). In the presence of neutral hydrazine, aldehyde adducts react further to form protonated hydrazones, e.g., CH3CH2CH=HNNH2+ from propanal. Using deuterated hydrazine (N2D4) and butanal, we demonstrate that the gas-phase ion chemistry of hydrazinium and carbonyls operates by the same mechanisms postulated for the reactions in solution. Calculations provide insight into specific steps and transition states in the reaction mechanism and aid in understanding the likely reaction process upon chemical or translational activation. For most carbonyls, rate coefficients for adduct formation approach the predicted maximum collisional rate coefficients, k similar to 10(-9) cm(3) molecule(-1) s(-1). Formaldehyde is an exception (k similar to 2 x 10(-11) cm(3) molecule(-1) s(-1)) due to the shorter lifetime of its collision complex. Following adduct formation, the process of hydrazone formation may be rate limiting at thermal energies. The combination of fast reaction rates and unique chemistry shows that protonated hydrazine can serve as a useful chemical-ionization reagent for quantifying atmospheric carbonyl compounds via CIMS. Mechanistic studies provide information that will aid in optimizing reaction conditions for this application.