A transformation pathway during thermal treatment of metastable Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases , , and was investigated during linear heating with two heating rates of 1.9 and 5 and during aging at 450 . The results showed that with a sufficient heating rate (5 in this case), the phase dissolves before the phase forms. On the other hand, for the slower heating rate of 1.9 , a small temperature interval of the coexistence of the and phases was detected. Volume fractions and lattice parameters of all phases were also determined.