Using phase plane analysis, the existence of travelling wave solutions to a system of reaction-diffusion equations describing a two-site, nonequilibrium, nonlinear sorption model is demonstrated. Singular perturbation methods are then applied to obtain approximations to the waveforms for Langmuir and Freundlich kinetics in the case of a large Peclet number or a large rate constant. The results are compared with other recent results where physical arguments are made to obtain similar, yet different, approximations, and their validity is examined.