Pressure-area isotherms of distearoylphosphatidylcholine-fatty acid mixtures at the air/water interface show a strong hysteresis in terms of higher pressure-area path during expansion compared with compression. The hysteresis is found to depend on the composition of the mixtures, length of the fatty acid chain and the barrier speed used in recording the isotherms. The origin of this non-equilibrium phenomenon is the slow pressure response of the composite monolayers during the compression and expansion half-cycles. This is attributed to the diffusion of fatty acid molecules into the lipid domains and the subsequent domain growth and coalescence occurring during compression and the reverse process during expansion. The enhanced hysteresis in the longer chain fatty acid composites at high barrier speeds allows an analysis of the kinetics of pressure equilibration.