Herein we report the synthesis of a library of thioalkyl- and thioaryl-pyridazinediones for thiol-based self-immolative release of cargo. A bisthioaryl-pyridazinedione is shown to be stable to serum protein albumin but unstable in intracellular conditions. A derivatised analogue underwent self-immolative degradation in cellular thiol conditions as evidenced by LC-MS/release of a turn-on fluorescence fluorophore; versatility of the thiol-pyridazinedione is demonstrated through synthesis of SMDC precursors that contain three different functional groups on the same central molecule.