The di(hydroperoxy)alkane adducts of phosphine oxides 1-9, Ph3 PO⋅(HOO)2 CMe2 , Cy3 PO⋅(HOO)2 CMe2 , Ph3 PO⋅ (HOO)2 CMeEt, Cy3 PO⋅(HOO)2 CMeEt, Cy3 PO⋅(HOO)2 CEt2 , Cy3 PO⋅ (HOO)2 C(CH2 )5 , Cy3 PO⋅(HOO)2 CMePh, (Ph2 P(O)CH2 CH2 P(O)Ph2 )⋅ ((HOO)2 CEt2 )2 , and Ph2 P(O)CH2 P(O)Ph2 ⋅(HOO)2 CMe2 , respectively, are synthesized and fully characterized by 1 H, 13 C, and 31 P NMR, and IR spectroscopies. Single crystal X-ray structures are reported for 3-9. Different one-pot synthetic pathways, starting from R3 P, R3 PO, R3 PO⋅H2 O, and R3 PO⋅H2 O2 are explored and discussed and a mechanism for the formation of the di(hydroperoxy)alkane adducts of phosphine oxides is suggested. The longevity of the adducts is tested by monitoring the oxidation of Ph3 P with quantitative NMR. The solubilities of the adducts in organic solvents are presented, and their applicability as stoichiometric oxidizing agents for the selective oxidation of sulfides to sulfoxides is reported.