The scattering length density of the AmB layer (1.07 × 10−6 Å−2 in H2O, 1.98 × 10−6 Å−2 in 66% D2O and 2.18 × 10−6 Å−2 in 100% D2O) shows that (23 ± 5) % v/v of ergosterol relative to AmB, or in total (0.54 ± 0.1) μmol m−2 ergosterol is incorporated, which corresponds closely to the amount of ergosterol originally present within the hlipid membrane (0.58 ± 0.1) μmol m−2. Thus, it is apparent that all the sterol is extracted from the membrane into the AmB sponge layer. Considering the molecular volumes of ergosterol [41] and AmB [27], the volume fraction of AmB inserted corresponds to approximately 1 AmB per 4 sterols originally in the membrane, while the lipid chains expand to fill the remaining space giving rise to the membrane thinning observed.