Abstract: Reversible pegylated drugs are provided by derivatization of free functional groups of the drug selected from amino, hydroxyl, mercapto, phosphate and/or carboxyl with groups sensitive to mild basic conditions such as 9-fluorenylmethoxycarbonyl (Fmoc) or 2-sulfo-9-fluorenylmethoxycarbonyl (FMS), to which group a PEG moiety is attached. In these pegylated drugs, the PEG moiety and the drug residue are not linked directly to each other, but rather both residues are linked to different positions of the scaffold Fmoc or FMS structure that is highly sensitive to bases and is removable under physiological conditions. The drugs are preferably drugs containing an amino group, most preferably peptides and proteins of low or medium molecular weight. Similar molecules are provided wherein a protein carrier or another polymer carrier replaces the PEG moiety.