A new sulfide crosslinkable resin was synthesized from condensation of dihydroxydiphenylsulfide with formaldehyde in a one-pot reaction system, and then purified and characterized. The tetrmethylol resin sets thermally to a very hard product. The thermosetting characteristics of the resin were investigated by four techniques: moisture evolution analysis, DSC, spectrorheometry, and thermogravimetry. The activation energy of the setting reactions was determined by these techniques; the values determined were 78.87, 14.56, and 144.48 kJ/mol as determined by the first three techniques, respectively. The suggested mechanism for the crosslinking reaction is also included. The thermally set resin showed outstanding thermal stability as found by thermogravimetric analysis technique.