Five new hydrogen peroxide adducts of phosphine oxides (p-Tol3PO·H2O2)2 (1), (o-Tol3PO·H2O2)2 (2), (o-Tol2PhPO·H2O2)2 (3), (p-Tol3PO)2·H2O2 (4), and (o-TolPh2PO)2·H2O2 (5), and the water adduct (o-Tol2PhPO·H2O)2 (6) have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The IR and 31P NMR data are in accordance with strong hydrogen bonding of the hydrogen peroxide. The mono- versus dimeric nature of the adduct assemblies has been investigated by DOSY NMR experiments. Raman spectroscopy of the symmetric adducts and the ν(O-O) stretching bands confirm the presence of hydrogen-bonded hydrogen peroxide in the solid materials. The solubilities in organic solvents have been quantified. Due to the high solubilities of 1-6 in organic solvents their 17O NMR spectra could be recorded in natural abundance, providing well-resolved signals for the P[double bond, length as m-dash]O and O-O groups. The adducts 1-5 have been probed regarding their stability in solution at 105 °C. The decomposition of the adduct 1 takes place by loss of the active oxygen atoms in two steps.