The time delay needed to renew the atmosphere inside the furnace of a thermogravimeter is a key parameter while studying the very early stages of chemical reactions involving a reactive gas. In this paper, we report the time evolution of oxygen partial pressure in a cylindrical horizontal furnace. The measurements have been carried out by means of the oxidation of nickel microparticles. The time that oxygen needs to reach the sample's position has been measured as a function of the furnace temperature and of gas flow rate. The values of time delay are shorter than expected at low flow rates due to gas interdiffusion, whereas are higher than expected at high flow rates due to partial thermalisation of gas. Gas interdiffusion has been computed by solving the mass transport equation in one dimension and selecting a suitable boundary condition at the end of the furnace. This boundary condition takes into account the abrupt reduction of section when gas flows through the exhausting hose. (C) 2003 Elsevier B.V. All rights reserved.