The mechanism of the shift, broadening and quenching of the ammonia inversion frequency with gas pressure has been a problem of lively interest for over seventy years. A simple quantum model of the ammonia molecule perturbed by collisions with ideal gas molecules displays the essential features of the experimental data for NH3 and for ND3. The model does not display the behaviour expected from theories of quantum localisation such as quantum state diffusion and decoherence. On the other hand, models of perturbed classical oscillators do display similar behaviour to our model. The quenching of the ammonia inversion transition cannot therefore be interpreted as spatial localisation of the wavefunction.