A comprehensive critical review of the phase fields, metastable modifications, solid solution ranges and phase transitions of monazite- and zircon-type REEXO4 (X = P, As, V) compounds is given. Monazite-type REEPO4 compounds are stable for REE = La to Gd and metastable for Tb to Ho; zircon-type members exist for REE = Gd to Lu, and Y, Sc. REEAsO4 compounds with monazite-type structure exist for REE = La to Nd, while zircon-type compounds are known for REE = Pm to Lu, and Y, Sc; no metastable arsenate members are known. The only stable monazite-type REEVO4 is LaVO4, but metastable members are known for REE = Ce to Nd. Zircon-type REEVO4 compounds are stable for REE = Ce to Lu, and Y, Sc, and metastable for REE = La. Solid solution series are complete only if minor size differences exist between REE3+ or X5+ cations in respective end-members. Phase transitions occur under pressure (zircon → (monazite →) scheelite) and at very low temperatures. The evaluation of the metastable phase fields and of naturally occurring members suggests that metastable modifications of REEXO4 compounds can occur in nature under certain conditions (formation at temperatures < ∼200–300°C; formation via hydrated precursor phases; stabilisation by various impurity cations).