The effect of the interphase/interface region on the creep and creep rupture behavior of thermoplastic (J2) composites was investigated. The mechanical properties of the J2 composites were altered by systematic changes in fiber surface characteristics. The three fiber surface variations investigated were: untreated AU4, electrochemical surface treated AS4(1) and (2), and surface treated/epoxy sized AS4CGP fiber. (AS4(1) and AS4(2) represent batch numbers.) For the same batch of the composites, the static mechanical properties, in general, ordered themselves from strong to weak as follows: AS4(2)/J2 > AS4CGP/J2 > AU4/J2. However, the creep rupture strength revealed a different ordering: AS4CGP/J2 > AS4(2)/J2 > AU4/J2. This indicates that good static mechanical properties may not dictate good long term mechanical performance. The creep rupture strength of the AS4(1)/J2 composites degraded about two times faster than the other three composites. Experimental results showed that the interphase/interface region did not affect the degradation rates of the creep rupture strength of the thermoplastic composites, but did influence the creep rupture strengths.