Compression isotherms illustrating metastable behavior in the three major films studied here. Films spread on a captive bubble at ambient temperatures were compressed either at 23°C [dipalmitoyl phosphatidylcholine (DPPC)] or 26°C [1-palmitoyl-2-oleoyl phosphatidylcholine (POPC); calf lung surfactant extract (CLSE)]. Approximate average rates of change in area were “fast,” ≥300 Å2 · molecule−1 · min−1, or “slow,” ≤1.8 Å 2 · molecule−1 · min−1. Black labels indicate conditions at which previous microscopic studies demonstrate the presence of the tilted-condensed (TC), liquid-expanded (LE), liquid-ordered (Lo), and collapsed bulk phase (9, 10, 12, 29). Colored arrows indicate the direction of progression along the curves.