The di(hydroperoxy)alkane adducts of phosphine oxides 1–9, Ph3PO⋅(HOO)2CMe2, Cy3PO⋅(HOO)2CMe2, Ph3PO⋅ (HOO)2CMeEt, Cy3PO⋅(HOO)2CMeEt, Cy3PO⋅(HOO)2CEt2, Cy3PO⋅ (HOO)2C(CH2)5, Cy3PO⋅(HOO)2CMePh, (Ph2P(O)CH2CH2P(O)Ph2)⋅ ((HOO)2CEt2)2, and Ph2P(O)CH2P(O)Ph2⋅(HOO)2CMe2, respectively, are synthesized and fully characterized by 1H, 13C, and 31P NMR, and IR spectroscopies. Single crystal X‐ray structures are reported for 3–9. Different one‐pot synthetic pathways, starting from R3P, R3PO, R3PO⋅H2O, and R3PO⋅H2O2 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 Ph3P 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. Crystal Cornucopia: When phosphine oxides are combined with ketones and aqueous hydrogen peroxide, di(hydroperoxy)alkane adducts form. Irrespective of the substituents at phosphorus, or the ketone, all adducts crystallize in the form of giant specimens that can easily be characterized by X‐ray crystallography. These novel peroxides are well‐defined with respect to their structure and composition. Furthermore, they are solid, soluble in organic solvents, and they represent active, but safe oxidizing agents with long shelf lives.