Lanthanum oxide is a white solid that is insoluble in water, but dissolves in acidic solutions.  absorbs moisture from air, converts to lanthanum hydroxide.
Lanthanum oxide has p-type semiconducting properties and a band gap of approximately 5.8 eV. Its average room temperature resistivity is 10 kΩ·cm, which decreases with an increase in temperature.  has the lowest lattice energy of the rare earth oxides, with very high dielectric constant, ε = 27.