More than four decades ago A. F. Andreev predicted theoretically the existence of quantum statesof 3He atoms on free surface of liquid 4He . In other words, at low temperatures, even forextremely dilute solutions, a substantial number of 3He atoms are adsorbed at the surface,forming 2D Fermi liquid. We therefore propose use of neutron reflection to study the 2D 3He layeradsorbed on liquid 4He. According to calculations , the reflected neutrons should exhibit oscillatorybehaviour caused by existence of 3He layer. One of the most interesting and important aims of theproposed experiment is to study the influence of temperature on the process of formation of theAndreev states.