The heterogeneous uptake of methylamine (MA) onto (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>HSO<sub>4</sub>, NH<sub>4</sub>NO<sub>3</sub> and NH<sub>4</sub>Cl was investigated using a Knudsen cell reactor coupled to a quadrupole mass spectrometer, in situ Raman spectrometer and theoretical calculations. Exchange reactions were observed between MA and NH<sub>4</sub>NO<sub>3</sub>, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, and NH<sub>4</sub>Cl were observed at 298 K. Simple acid-base reaction for MA was found taking place on NH<sub>4</sub>HSO<sub>4</sub>. CH<sub>3</sub>NH<sub>3</sub>NO<sub>3</sub> and CH<sub>3</sub>NH<sub>3</sub>Cl are not stable at low pressure and have higher dissociation vapor pressure than methylammonium sulfate. The observed uptake coefficients of MA on (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>HSO<sub>4</sub>, NH<sub>4</sub>NO<sub>3</sub> and NH<sub>4</sub>Cl at 298 K were measured to be 6.30±1.03×10<sup>−3</sup>, 1.78±0.36×10<sup>−2</sup>, 8.79±1.99×10<sup>−3</sup> and 2.29±0.28×10<sup>−3</sup>, respectively. A linear free energy relationship was found for the heterogeneous reactions between MA and NH<sub>4</sub>Cl, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and NH<sub>4</sub>NO<sub>3</sub>. Namely, the natural logarithm of uptake coefficients of MA on these ammonium salts is linearly related to the electrostatic potential of the H atom in the NH<sub>4</sub><sup>+</sup> group.