Abstract The dielectric and dynamic mechanical relaxation behaviours of the thermoplastic polyimide, NEW-TPI, have been investigated from 150 to 350°C, which spans the glass transition region. Dynamic modulus at 1 Hz is about 2.2 GPa below the glass transition temperature, T g, decreasing to 0.02 GPa in amorphous NEW-TPI, and 0.15 GPa in semicrystalline NEW-TPI, above T g. Dielectric constant at 10 kHz is about 3.21 below T g, increasing to 3.44 in amorphous NEW-TPI, and 3.33 in semicrystalline NEW-TPI, above T g. Williams-Landel-Ferry (WLF) plots for amorphous and one representative semicrystalline NEW-TPI were constructed from thermal, dynamic mechanical and dielectric relaxation data but the data could not be fitted to a single master curve. Dielectric relaxation intensity, Δε = ε s − ε ∞, was shown to be structure sensitive above T g. For both semicrystalline and amorphous NEW-TPI, the relaxation intensity decreases as temperature increases. This implies that Δε has the same temperature dependence for the semicrystalline sample compared to the quenched amorphous polymer. This trend is different from that observed in either poly(ether ether ketone) or poly(phenylene sulphide). Our results confirm thermal analysis of NEW-TPI and show that NEW-TPI has a very small amount of tightly bound, or rigid, amorphous material, which relaxes completely within a narrow temperature range just above the T g of the less tightly bound, or mobile, amorphous material.