Molybdic acid refers to solid, hydrated forms of molybdenum trioxide and species in aqueous solution.
The simplest solid form, the monohydrate, is MoO₃.H₂O, the dihydrate MoO₃.H₂O is also known. The solid state structure of MoO₃.H₂O consists of layers of octahedrally coordinated MoO₃.H₂O units where 4 vertices are shared. The dihydrate has the same layer structure with the extra H₂O molecule intercalated between the layers. In aqueous solutions of acidified molybdate salt at low concentration molecular O₃Mo.3H₂O has been characterised spectroscopically.
The salts of molybdic acid are called molybdates.