Abstract The method used to fill a chemical heat pump reactor with its porous reactive medium governs the heat and mass transfer in the reactor and hence its performance in terms of heat, power and capacity. The filling operation consists of mixing the salt, which takes place in the solid-gas reaction, and an inert graphite binder, and in confining this mixture to obtain a given apparent density. Depending on the binder properties and the mixing technique, the values (experimentally obtained) of the effective conductivities, λe, of the heat transfer coefficients, hsw, at the exchanger wall and of the permeabilities, k, cover a very large range: 0.2 W m −1 K −1 ⩽λ e ⩽40 W m −1 K −1 15 W m −2 K −1 K −1 ⩽h sw ⩽15,000 R m −2 K −1 , 0.03 × 10 −12 m 2 ⩽k⩽30 × 10 −12 m 2 . For some simple cases in which one limitation predominates over all the others, a simple relation has been set up between the reactor's power output per unit volume and a single filling parameter.