An efficient liquid-phase synthesis of 3,5-disubstituted 1,3,5-thiadiazinane-2-thione derivatives (THTT) is described using soluble polymer support polyethylene glycol (PEG) 5000 via one-pot condensation of PEG-bound free amino acid or PEG-bound tripeptide, a dithiocarbamate and formaldehyde under mild conditions. This procedure affords the target compounds in good yields and high purity with a facile work-up procedure. This synthetic approach has also allowed the efficient preparation of new compounds endowed with two THTT rings linked through the N-3 nitrogen atoms. The structures of all synthesized compounds were unequivocally established by standard spectroscopic techniques.