Positron lifetime spectra of Ti50Al50, Ti50Al48V2, Ti50Al48Ag2 alloys and annealed Ti, Al, Ag, V metals were measured. The electron densities in the bulk and defects of the alloys were calculated by positron lifetime parameters. The poor ductility of binary TiAI alloy is related to low free electron densities in the bulk and the grain boundaries of the alloy. When V are added into Ti-rich TiAI alloy, V atoms will provide more free electrons than both Al and Ti atoms to participate in metallic bonds, thus increasing the electron densities in the bulk and the grain boundary simultaneously. Ag additions appear to have an effect similar to V additions. Both V and Ag are benefit elements in enhancing the ductility of TiAl alloys.