The aim of this work is to develop an experimental device for the measurement of porous media textural properties during physical or chemical transformation. The method is based on gas tracer experiments. The system investigated consists of a spherical reactor containing a spherical porous media. To avoid surface stresses, there is no contact between the sample and the reactor. We called the cell a particle in box, or "PIB". The porosity of the solid and the gas effective diffusivity were determined using a parametric identification method. The model is based on the perfectly stirred reactor approach. The PIB methodology was first checked at room temperature to determine the solid porosity and the effective diffusion coefficient. Experiments were then performed during transformation of a sample composed of a mixture of hydroxylapatite particles, glycerol, and water. In situ determination of the porosity and diffusivity from room temperature to 573 K (300 degrees C) was demonstrated.