We have examined the effects of lysophosphocholines on dipalmitoylphosphocholine (DPPC) monolayers compressed through the liquid-expanded/liquid-condensed coexistence region. Isotherms and the shapes of liquid-condensed domains revealed three consistent behavioral regimes based on lysolipid chain length. Group I lysolipids (C-8 through C-12) desorb readily from the interface upon compression, as shown by isotherm results, and produce little effect on DPPC domain shape. Group II lysolipids (C-14 and C-16) remain kinetically trapped at the interface upon compression and show hints of line activity toward DPPC domains. Both group II and group I lysolipids exhibit a high-pressure transition in domain shape reported previously for mixtures of DPPC and charged surfactants. Finally, group III lysolipids (C-18 through C-22), which are insoluble, strongly affect DPPC isotherms as well as domain shapes; the lysolipid exhibits significant line activity, resulting in noncompact, extended domains. All of these results are discussed with respect to the solubility of the lysolipid component as well as the impact of intermolecular forces within the monolayer.