Thermal stability and thermal expansion properties are reported for several high performance polymers which contain carbonyl and/or ether linking groups designed to increase molecular mobility while retaining high temperature properties. The polymers studied include poly(ether ether ketone), PEEK, and three related novel thermoplastic polyimides : NEW-TPI, LARC-CPI, and LARC-IA. Thermogravimetric analysis showed that LARC-CPI exhibited major weight loss by about 450 degrees C, and both PEEK and LARC-CPI lost 50% of their initial weight by 750 degrees C. Thermomechanical analysis (t.m.a.) was used to determine linear coefficients of thermal expansion (CTE). All materials showed a large chan transition temperature (T-g). Bulk linear CTEs ranged from 54-70 x 10(-6) degrees C-1 below T-g, to 105-200 x 10(-6) degrees C-1 above T-g. These CTEs are larger than those typically observed for more rigid chain polyimides. Using a two-phase model, the amorphous phase CTE was deduced for the semicrystalline materials. From t.m.a. data, amorphous phase CTEs above T-g ranged from 120-245 x 10(-6) degrees C-1. Small angle X-ray scattering (SAXS) was used to examine changes in long period with temperature for PEEK and NEW-TPI. Below T-g, the long period did not change much while above T-g, larger increases in the long period were observed. These increases in the long period with increasing temperature may be the result either of large amorphous phase expansion, or possibly of melting of thin lamellae.