The influence of post-weld heat-treatment and of residual silicon in aluminium on the mechanical properties of dissimilar friction joints between titanium and aluminium was investigated. Although joint tensile strength and bend test properties were drastically reduced following post-weld heat treatment, the responses of Ti/h.p. Al and Ti/c.p. Al joints were quite different. The tensile strength and bend test properties of Ti/h.p. Al joints were markedly decreased by heat-treatments involving shorter holding times at lower temperatures. Joint failure in post-weld heat-treated joints was associated with Al3Ti formation at the bondline region. The growth rate of the Al3Ti intermetallic layer at the joint interface was much faster in post weld heat-treated Ti/h.p. joints. More than 20 at %Si segregated in the region between the titanium substrate and the Al3Ti intermetallic phase in heat-treated Ti/c.p. Al joints. It is suggested that silicon segregation retards Al3Ti formation by acting as a barrier to titanium and aluminium diffusion at the joint interface.