Thermoplastic polyimides are a relatively new class of polymers that exhibit high-temperature stability and are useful in composite applications. One such material is Avim-id® K3B reinforced with Magnamite® IM7 graphite fibers. This composite system exhibits excellent strength and toughness, retains its strength and toughness after prolonged exposure to elevated temperatures, and resists microcracking at extremely low (liquid nitrogen) temperatures. Further characterization of IM7/K3B thermoplastic composites is focused on evaluating hygrothermal effects on the material properties and behavior. The environmental conditioning test matrix includes three temperatures (20, 40, and 80°C) and four relative humidity levels (75, 85, 97, and 100%). Observations and conclusions from these studies include the following: (1) the moisture diffusivity of IM7/K3B has a classic Arrhenius dependence on temperature; (2) the moisture saturation level depends on the relative humidity level to the power of 1.34 with a maximum value of 0.55% by weight; (3) the glass transition temperature Tg is lowered with moisture absorption but is recovered when the sample is redried; (4) the intralaminar fracture toughness GIc remains constant after extensive hygrothermal conditioning; (5) the diffusion kinetics are Fickian in general, except for a few non-Fickian anomalies that are related to development of transverse microcracks.