This study investigates the interfacial reactions between eutectic SnBi solder and electroless Co(W,P) layer with various crystallinities via the liquid-state aging (250°C, up to 1 hr) and solid-state aging (120°C, up to 1000 hrs) so as to explore the diffusion barrier characteristics of electroless Co(W,P) to SnBi. In the samples containing SnBi/amorphous Co(W,P) subjected to liquid-state aging, a polycrystalline P-rich layer containing mixed IMC phases formed in between solder and Co (W,P) whereas the lath-like CoSn3 intermetallic compound (IMC) spalled into the solder after 1-hr aging. In the samples containing SnBi/polycrystalline Co(W,P) subjected to liquid-state aging, lath-like CoSn3 IMCs formed with spallation and an amorphous W-rich layer was found in between IMCs and Co(W,P). As to the samples subjected to solid-state aging, reactions of solder and Co(W,P) generated a thick, lath-like IMC layer in between solder and Co(W,P) regardless of the crystallinity of Co(W,P). Analytical results illustrated that the electroless Co(W,P) layer is in essential a sacrificial-type barrier to SnBi solder.