Surface viscosities of phospholipid/steroid mixtures at the air-water interface were measured by means of an oscillating pendulum. Phospholipids studied included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2dipalmitoyl-sn-glycero-3-phosphodimethylethanolamine (DPPDME); 1,2-dipalmitoyl-sn-glycero-3-phosphomonomethylethanolamine (DPPMME); 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE); 1,2-dipalmitoyl-snglycero-3-[phospho-rac-(1-glycerol)] (DPPG); 1-palmitoyl-2-elaidyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2linelaidyl-sn-glycero-3-phosphocholine. Each saturated phospholipid was studied in the presence of cholesterol. DPPC was also investigated in the presence of 5-androsten-3fl-ol; cholestanol; 5-cholestene; 5cΒ’-cholesten-3fl-ol methyl ether; coprostanol; 7,(5~)-cholesten-3fl-ol; desmosterol; epicholestanol; lanosterol and lophenol. The surface viscosities of the trans-unsaturated phosphatidylcholines (PC) were undetectable by this method and in this respect they resembled cis-unsaturated PC. The surface viscosities of saturated phospholipids were very high but were reduced by low concentrations of steroids. Interpretation of the results for DPPC/cholesterol mixtures indicates that DPPC functions at the air-water interface as a one-dimensional linear aggregate. At 50 mN/m and 22Β°C the average structure contains approximately 300 DPPC molecules. DPPDME acts similarly but DPPMME, DPPE and DPPG differ from DPPC in their response to cholesterol. All of the steroids examined except 5-androsten-3fl-ol and the lanosterol mixture paralleled cholesterol in their interaction with DPPC an indication that phospholipid/steroid interactions modifying surface viscosity have less stringent requirements for the steroid structure than interactions measured as condensation in surface area.