Sintering ceramics involves neck formation, densification and eventually grain growth. An analytical model is developed to describe the effects of pore fraction, grain conductivity, average grain size and the contact area between particles due to neck formation on the thermal conductivity of the green or partially sintered ceramic. It has been tested on partially sintered ceramics of hydroxyapatite, zirconia and alumina fired with conventional and modern rapid heating techniques. The green bodies containing at least 40% porosity yield conductivity values in the range 0.2 to 0.9 Wm-1K-1. Neck formation in the initial stage of sintering increases the conductivity values by a factor of 2 or more. Further increase is achieved by densification, well described by Landauer’s relation as part of the model with close agreement to experiment for hydroxyapatite ceramics containing 40 to 5% porosity. In the case of alumina, grain growth must be taken into account as well. Thus, characteristic paths for sintering can be established in a plot of thermal conductivity versus porosity related to the predominant mechanisms. Such information on the thermal conductivity / diffusivity of partially sintered ceramics will also be relevant as input data for numerical modelling of different sintering techniques applied to a ceramic green body.