aliphatic carboxylic acids with D2O as an inexpensive deuterium source. The use of Ph3P as an O‐atom transfer reagent can facilitate the deoxygenation of aromatic acids, while Ph2POEt is a better O‐atom transfer reagent for aliphatic acids. The highly precise deoxygenation of complex carboxylic acids makes this protocol promising for late‐stage deoxygenative deuteration of natural product derivatives