It is known that the rotational tunnelling spectra of NH4+ ions in (NH4)(x)K1-xI show of significant changes with increases in the NH4 ion concentration. This Letter reports the results of high-resolution neutron inelastic measurements of (NH4)(0.02)K0.98I, (NH4)(0.02)(K0.9Rb0.1)(0.98)I and (NH4)(0.1)Rb0.9I, and shows unequivocally that the observed changes in the tunnelling spectra of NH4+ ions in (NH4)(x)K1-xI with x arise because of increase in both the NH4+-NH4+ interaction and in the strain field in the lattice. The effect of changes in the strain field on the tunnelling spectra are qualitatively different from. those arising from a change in NH4+-NH4+ interaction.