Measurements of in situ atmospheric water absorption and desorption in ionic liquids (ILs) (1-ethyl-3-methylimidazolium tetrafluoroborate [C(2)C(1)im][BF4], 1-butyl-3-methylimidazolium acetate [C(4)C(1)im][OAc], and 1-butyl-3-methylimidazolium chloride [C(4)C(1)im] [Cl]) were made using a gravimetric microbalance at temperatures ranging from 283.15 to 315.15 K and relative humidity (RH) 0-70% at 101 kPa. Solubility data were well correlated using the nonrandom two-liquid (NRTL) activity model, and time dependent concentration data were used to determine the binary diffusion coefficients using one- and two-dimensional transport models of water in the IL-water systems. The solubility of water was highest in [C(4)C(1)im][OAc] (77.5 mol %), followed by [C4C1 im][Cl] (68.6 mol %), and [C(2)C(1)im][BF4] (19.5 mol %) at equivalent conditions (303.15 K and 25.00% RH). The diffusion coefficients in order of increasing relative humidity ranged from 1.3 x 10(-10 )to 2.8 x 10(-11 )m(2)/s for [C(2)C(1)im][BF4], from 8.8 x 10(-12) to 3.9 x 10(-11) m(2) /s for [C(4)C(1)im][OAc], and from 4.5 x 10(-12) to 2.8 x 10(-11) m(2) /s for [C(4)C(1)im][Cl]. Heats of absorption were calculated and ranged from 39 to 44 kJ/mol for [C(2)C(1)im][BF4], from 47 to 45 kJ/mol for [C(4)C(1)im][OAc], and from 55 to 45 kJ/mol for [C(4)C(1)im][Cl] with increasing water mole fraction of 0.3-0.8. The water diffusivity increases with increasing water concentration in both [C(4)C(1)im][OAc] and [C(4)C(1)im][Cl] with respect to decreasing viscosity and heats of absorption. However, the diffusivity of water in [C(2)C(1)im][BF4] decreases with increasing water concentration with respect to decreasing viscosity and increasing heats of absorption. Diffusing radius calculations using the Stokes-Einstein relationship support the hypothesis that a few water molecules through hydrogen bonding form clusters with the [OAc] and [Cl] anions, but much larger water/BF4 clusters/networks are occurring in the [C(2)C(1)im] [BF4] system which increase in size with increase in water concentration.