We have studied analytically and numerically a nonlinear diatomic lattice with a cubic nearestneigbbor interaction potential. Our system is a one-dimensional chain of pairs of atoms interacting through a «hard» interaction, each pair being bound to the neighboring pairs by a «soft» interaction. This is a simple model for hydrogen-bonded molecular chains, like the spines in an a helix. We have used a multiple-scale reductive perturbative technique to transform the equations of motion, and derived a nonlinear Schrodinger equation describing the time evolution of localized solitonic excitations