Bobjonesite, V⁴⁺ O (SO₄) (H₂O)₃, is a new mineral species from Temple Mountain, Emery County, Utah, U.S.A. It occurs as blue-green crusts and efflorescences in fractures in a fossil (Triassic) tree; individual crystals are <<1 mm and are intimately intergrown. Bobjonesite hydrates very easily, and is unstable in all but the driest atmosphere. Its structure was determined on a crystal of bobjonesite; however, the physical properties, optical properties and X-ray powder-diffraction pattern were recorded on the synthetic equivalent, and an electron-microprobe analysis was not possible. Bobjonesite has a pale blue streak, a vitreous luster and no observable fluorescence under ultraviolet light. It has no cleavage or parting. The Mohs hardness is ~1, and the calculated density is 2.28 g/cm³. Bobjonesite is biaxial positive, with α 1.555(2), β 1.561(1), γ 1.574(2), 2V(obs.) = 72(1)°, 2V(calc.) = 69°; it is non-pleochroic, X = b, Y ≈ a, Z ∧ c ≈ 19° (in β obtuse). Bobjonesite is monoclinic, space group P2₁/n, cell dimensions from single-crystal data: a 7.3940(5), b 7.4111(3), c 12.0597(9) Å, β 106.55(1)°, V 633.5(1) ų, Z = 4. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are as follows: 5.795(100)(002), 3.498(90)(112), 3.881(48)(1̅03), 5.408(37) (101), 4.571(20)(012), 6.962(11)(1̅01) and 6.254(11)(011). The chemical formula was derived from crystal-structure analysis; the end-member formula is V O (SO₄) (H₂O)₃. The crystal structure of bobjonesite was refined to an R index of 3.6% for 1105 observed (| F_o | > 5>F) reflections measured with an automated four-circle X-ray diffractometer using MoKα X-radiation. There is one V site occupied by V⁴⁺ and surrounded by three O atoms and three (H₂O) groups in an octahedral arrangement, with one short vanadyl bond (1.577 Å), four similar equatorial bonds (<2.022 Å>), and one longer V–O bond (2.278 Å) trans to the vanadyl bond. The structure consists of isolated [V⁴⁺₂ O₂ (H₂O)₆ (SO₄)₂] clusters linked by hydrogen bonds.