In this paper we report experimental and theoretical results on total inelastic cross-sections for NH3molecules colliding with H2at room temperature. The use of time resolved infrared-infrared double resonance allows one to follow the population relaxation for different energy levels of NH3. A CO2pulsed pump laser tuned to a line in thev2band of NH3induces a transient transfer of population and the return to equilibrium is probed with a c.w. diode laser turned to lines in thev4band of NH3. The time evolution of the probe absorption for different pressures of the two gases is then related to the inelastic cross-sections of the states involved. The so-determined cross-sections for the (j= 5,k= 4, ε = +1) and (j= 1,k= 1, ε = -1) states of NH3are respectively 27 Å2and 31 Å2. We have also calculated inelastic cross-sections using a semi-classical model and anab initiopotential energy surface for different values of (j, k, ε) (i.e.j= 1, 2, 3,k= 1, 2, 3 and ε = ±1) and ofj2(total angular momentum of H2). These calculated values are in agreement with the experimental results.