New ferrites ErMFe2O5 (M = Li, Na, K) were synthesized from erbium and iron(III) oxides and lithium, sodium, and potassium carbonates by solid-state annealing. According to X-ray powder diffraction, these compounds crystallize in the orthorhombic system with the following unit cell parameters: ErLiFe2O5, a = 10.510 A, b = 10.776 A, c = 14.270 A, V 0 = 1616.16 A3; Z = 16, V subcell 0 = 101.01 A3, ρX = 6.01 g/cm3, ρpycn = 5.97 ± 0.05 g/cm3; ErNaFe2O5, a = 10.519 A, b = 10.785 A, c = 15.510 A, V 0 = 1759.56 A3, Z = 16, V subcell 0 = 109.90 A3, ρX = 5.77 g/cm3, ρpycn = 5.72 ± 0.08 g/cm3; ErKFe2O5, a = 10.050 A, b = 11.320 A, c = 15.480 A, V 0 = 1937.33 A3, Z = 16, V subcell 0 = 121.08 A3, ρX = 5.46 g/cm3, ρpycn = 5.41 ± 0.04 g/cm3.