The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by healing-cooling cycles in the temperature range from 50 to 600 degrees C. YCr1-xPxO4 started releasing oxygen as heated from 50 degrees C under ambient atmosphere, with reduction of Cr-V to Cr-IV, while the reduced YCr1-xPxO4-delta phase was significantly reoxidized via absorbing oxygen by cooling to 50 degrees C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-xPxO4-delta phases were stabilized by forming linking polyhedral (Cr2O76-)-O-IV via corner sharing between oxygen-deficient (CrO32-)-O-IV and adjacent (CrO44-)-O-IV. YCr1-xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol(-1) probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.