The invention provides modified nucleotide or nucleoside molecule comprising a purine or pyrimidine base and a ribose or deoxyribose sugar moiety having a removable 3'-OH blocking group covalently attached thereto, such that the 3' carbon atom has attached a group of the structure --O-Z wherein Z is any of --C(R')2-O--R'', --C(R')2-N(R'')2, --C(R')2-N(H)R'', --C(R')2-S--R'' and --C(R')2-F, wherein each R'' is or is part of a removable protecting group; each R' is independently a hydrogen atom, an alkyl, substituted alkyl, arylalkyl, alkenyl, alkynyl, aryl, heteroaryl, heterocyclic, acyl, cyano, alkoxy, aryloxy, heteroaryloxy or amido group, or a detectable label attached through a linking group; or (R')2 represents an alkylidene group of formula .dbd.C(R''')2 wherein each R''' may be the same or different and is selected from the group comprising hydrogen and halogen atoms and alkyl groups; and wherein said molecule may be reacted to yield an intermediate in which each R'' is exchanged for H or, where Z is --C(R')2-F, the F is exchanged for OH, SH or NH2, preferably OH, which intermediate dissociates under aqueous conditions to afford a molecule with a free 3'OH; with the proviso that where Z is --C(R')2-S--R'', both R' groups are not H.