The enhanced permeability of flat lipid bilayer membranes at their gel to liquid-crystalline (LC) phase transition has been explored using coarse-grained molecular dynamics. The phase transition temperature, T-m, is deduced by monitoring the area per lipid, the lipid lateral diffusion constant, and the lipid lipid radial distribution function. We find that a peak in the permeability coincides with the phase transition from the gel to LC state when lysolipid is present. This peak in permeability correlates with a jump in the area per lipid near the same temperature as well as increased fluctuations in the lipid bilayer free volume. At temperatures above T-m, the permeability is only slightly dependent on the amount of lysolipid present. The increased free volume due to the "missing tail" of the lysolipid is partially compensated for by a decrease in area per lipid as the amount of lysolipid increases. We also found that in the coarse-grained model a small amount (<= 15 mol %) of lysolipid stabilizes the gel phase and increases the phase transition temperature, while a larger amount of lysolipid (20 mol %) reduces T-m back to that for pure DPPC, and bilayers consisting of >= 30 mol % lysolipid did not form a gel phase but still exhibited a peak in permeability near T-m in for pure DPPC.