Cholesterol levels in membranes of cellular organelles varies immensely. For example, cholesterol in the plasma membrane (PM) is 35¿45% of the lipid content , while in the endoplasmic reticulum, the site of cholesterol synthesis, it is only 1¿10%. Further, the distribution of cholesterol across the PM is thought to be asymmetric since its outer leaflet is enriched in saturated lipids. These lipids are preferred by cholesterol due to better packing and water-shileding properties compared to unsaturated lipids. Here we propose to investigate the flipping and desorption/absorption behavior of cholesterol in supported flat membranes that mimic the outer PM using time resolved neutron reflectivity. MD simulations suggest that cholesterol flip-flop in these membranes is in the tens of minutes while in purely unsaturated or saturated membranes it is in the millisecond range.