The first vapor liquid equilibrium (VLE) measurements for the binary systems of ammonia (NH3) and three imidazolium-based ionic liquids (ILs) have been successfully measured using a gravimetric microbalance. ILs 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)C(1)im] [PF6]), 1-butyl-3-methylimidazolium tetrafluorob orate ([C4C1MI] [BF4]), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(2)C(1)im][NTf2]) were measured at temperatures of 283.15, 298.15, 323.15, and 348.15 K and at pressures up to 0.7 MPa using the new Hiden XEMIS gravimetric microbalance. The VLE data were correlated using the Peng-Robinson equation of state and the Non-Random Two Liquid (NRTL) activity coefficient models. Both models are in excellent agreement with the experimental data. The Fickian diffusivities of NH3 in imidazolium-based ILs were obtained fitting experimental concentration to the one-dimensional (1D) mass diffusion equation, and found to be about 3 to 5 times lower than the diffusion of NH3 in water (H2O). The semitheoretical Stokes-Einstein equation was used to model diffusivities and to obtain the diffusing radius of NH3 in imidazolium-based ILs.