To aim at a better understanding of the uranium isotope exchange reaction between gaseous UF6 and solid UF5 experiments were done with natural UF6 gas and solid UF5 containing 3% 235U under different pressures of UF6. The experimental results suggest a two-process reaction with an initial rapid increase of 235UF6 in the gas phase followed by its slight and gradual increase. A rate equation based on a collision model is given for the two-process reaction which includes a primary exchange reaction on the solid surface and a secondary reaction participated by underlying UF5 molecules. An analytical solution is provided for both of 235UF6 concentration in the gas phase and 235UF5 concentration on the solid surface, which is useful for determining the parameters characterizing the exchange reaction. A numerical analysis is also made to evaluate the influence of gas samplings. A remarkable agreement is found between the particle sizes of UF5 estimated from the reaction parameter and from the direct observation ...