Change in the R-phase transformation behavior with the increase of Fe/Co content in Ti-(50-x)Ni-xFe and Ti-(50-x)Ni-xCo alloys has been investigated through thermal property measurements and microscope observations in order to clarify the relation among the stability of the B2-type structure, premartensitic phenomena, and the R-phase transformation. It was found that with increasing Fe/Co content, the R-phase transformation behavior approaches a second-order like one. The transformation product formed very close to the second-order is composed of nanoscale domains, and its Debye temperature is low compared with the R-phase formed by obvious first-order transformation.