The addition of secondary phosphine oxides to tetramesityldisilene and -digermene results in the mild, partial reduction of the P(V) centre of the organophosphorus oxide to P(III) to yield disilyl and digermyl phosphinite derivatives and illustrates the potential of ditetrelenes to serve as mild reducing agents under ambient conditions. An analogous reaction happens with <i>H</i>-phosphonates. Mechanistic experiments, including deuterium-labelling, kinetic isotope effect (KIE) and variable time normalization analysis (VTNA) experiments, reveal that the 1,3-PH addition likely proceeds through a stepwise reaction pathway with the organophosphorus oxide acting as the nucleophile towards the ditetrelene. Furthermore, a facile exchange between the R₂PO- moiety on the digermylphosphinite with the R₂PO moiety of phosphine oxides was discovered and likely proceeds through a direct substitution mechanism.