Phosphotriester compound of general formula (I); ##STR1## wherein R is a radical --(CH.sub.2).sub.n --S--X in which X is a radical (a) or --S--U, Z being O or S, and Y and U denote an alkyl, aryl or osidic radical, optionally substituted in particular by an OH, SH or NH.sub.2 group, and n is from 1 to 4, preferably 1 or 2, and Nu is a radical consisting of a biologically active compound residue or the dephosphorylated residue of a compound that is biologically active when it carries a phosphate or phosphonate group.