Membrane thickness is thought to play a key role in protein function. Thus understanding the cell’s ability to modulate the thickness of its membranes is essential in elucidating the structure/ function relationship in biological membranes. We have investigated the influence of cholesterol on the structure of “thin” (diC14:1PC) and “thick” (diC22:1PC) phospholipid bilayers using oriented multibilayers and small angle neutron di raction. Neutron contrast variation was used to determine the structure factors and the distribution of water across the bilayers. We found that in response to cholesterol, bilayer thickness changed in a similar fashion in both systems. The thickening of bilayers was rationalized in terms of cholesterol’s ordering effect on the lipid’s acyl chains, which dominates over the other option of rectifying the hydrophobic mismatch, surprisingly even in the case of diC22:1PC and cholesterol. ; submission_instructions: CISTI - could you please repair the author list? ; peer reviewed: yes ; NRC Pub: yes ; system details: machine converted author identifier PE to PID, February 2012