The phase variations of AlH emission lines in LPV spectra were investigated using a model of the post-shock relaxation zone. Conditions at the shock were linked to position in the atmosphere and phase by a global, isothermal shock model. In the relaxation zone model, 78 species were considered, including e('-), H, H('+), H('-), H(,2), H(,2)('+), He, He('+), He('++), M, M('+), and M('++), where M is one of 23 metals. The model was applied at densities (LESSTHEQ) 10('-10.5), where the shocks were estimated to be optically thin, and molecular species were assumed to play a negligible role in determining the thermodynamic state of the gas. Calculations were performed for a 1.2 M(,(CIRCLE)) star, with P = 332('d), R = 225 R(,(CIRCLE)), and Q = 0.108 (fundamental mode pulsation).;At densities -12.5) of the atmosphere, relaxation occurs fast enough that the temperature profile of a given mass shell approaches the isothermal case.;The temperature of the atmosphere increases with radius.;The observed variations in AlH emission with phase are not caused solely by variations in the AlH formation rate, which decreases rapidly with decreasing density. Instead, the emergence and increased prominence of the emission lines toward minimum light are caused by the increasing transparency of the rising shock. Following minimum light, the declining formation rate diminishes the AlH emission lines, preventing their detection in the upper shock; this is consistent with the absence of the lines around maximum light.;Origin of these emission lines behind the rising lower shock causes a blue-shift in their velocities, which has been observed. Any AlH absorption features which are detected should be red-shifted.