2,4,8,10‐Tetraoxa‐3,9‐diphosphaspiro[5.5]undecane‐3,9‐disulfide (I) has been prepared by the treatment of 3, 9‐dichloro‐2,4,8,10‐tetraoxa‐3,9‐diphosphaspiro[5.5]undecane (II) with hydrogen sulfide. The reactions of I with thiocyanates to give esters of 3,9‐dimercapto‐2,4,8,10‐tetraoxa‐3,9‐diphosphaspiro[5.5]undecane‐3, 9‐disulfide (III) and with isocyanates and isothiocyanates to give P‐carbamoyl and thiocarbamoyl derivatives of I were investigated. I was also found to condense with chloral to produce an α‐hydroxy‐thiophosphonate (XXVIII), but similar reactions with other aldehydes failed to yield analogous products.