The positron annihilation lifetime (PAL) of a series of copolyimides and copolyamides with microphase-separated structures was measured to investigate the effects of different hard-segment polymers on the PAL properties of soft-segment domains of poly(dimethyl-siloxane) (PDMS) and poly(ethylene oxide) (PEO). The lifetime ( 3 ) and intensity (I 3 ) of the long-lived component are given as a function of the PDMS or PEO content for a series of copolymers, of which the density roughly obeys the additive rule except for the PDMS-segmented copolyamides. The PDMS-segmented copolyimides and copolyamides show much smaller I 3 values than those estimated from the additive rule. The lifetime distribution of the long-lived component for the PDMSsegmented copolyamides is composed of two components. The longer-lifetime component is attributed to pure PDMS domains, and the shorter-lifetime component is attributed to the polyamide domains, intermediate phases, and PDMS domains containing small amounts of short amide blocks. Despite the high PDMS content, the latter component is rather large. Thus, the positronium formation in the PDMS domains of the copolyimides and copolyamides is effectively reduced. This can be explained by the combination of the difference in the electron affinity of the PDMS and polyimide or polyamide segments and the incomplete phase separation. The PEO-segmented copolyimides show much smaller I 3 values than those predicted from the additive rule. This is likely attributable to the effects of the intermediate phases.