The present invention relates to novel heterocyclic pro-fragrances, preferably oxazolidines, tertahydro-1,3-oxazines, thiazolidines, or tetrahydro-1,3-thiazines, more preferably oxazolidines, or tertahydro-1,3-oxazines, most preferably oxazolidines, which are capable of sustained release of fragrance raw material ketones and aldehydes and to fragrance delivery systems which comprise said pro-fragrances.