Abstract We present a theory of unbinding transitions for membranes that interact via short and long receptor/ligand bonds. The detail of unbinding behaviour of the membranes is governed by the binding energies and concentrations of receptors and ligands. We investigate the unbinding behaviour of these membranes with Monte Carlo simulations and via a comparison with strings. We derive the scaling laws for strings analytically. The exact analytical results provide scaling estimate for membranes in the vicinity of the critical point.