The thermal conductivity of an active carbon bed in vacuum, helium, neon and argon has been measured at temperatures up to 200 degrees C. A modified version of the Zehner-Bauer model is developed which successfully describes the results. The model is expanded to include the variation in grain conductivity due to the adsorbed phase when the bed is exposed to an adsorbable gas. The expanded model is verified by conductivity measurements with ammonia at up to 11.3 bar pressure and 25% mass concentration. Conductivities are predicted to +3.1%, -2.5%. Bed conductivities with ammonia are typically 0.165 W m(-1) K-1, with minor changes due to concentration, but the grain conductivity varies between 0.85 and 1.25 W m(-1) K-1 as a non-linear function of concentration.