The sterol composition of Spheciospongia vesparia from the Caribbean Sea is reported. Three groups of sterols, Delta(5), Delta(0), Delta(5,7) were found. The isolated 5 alpha,8 alpha-epidioxy sterols appeared to be predominantly, if not exclusively, artifacts. The auto-oxidation of native Delta(5,7)-sterols to 5 alpha,8 alpha-epidioxy sterols has been investigated. A transformation of dietary Delta(5)-sterols to Delta(5,7)-sterols has been demonstrated in Spheciospongia vesparia.