The diffusion coefficient of radioactive radon (Rn) in a porous medium was determined in this study by measuring the time-dependent diffusion of radon through a thin slab of the material. The diffusion process was required to reach a steady state so as to obtain a good estimate of the diffusion coefficient. A numerical method using the nonlinear least-squares and Marquardt's method (NLSM) was proposed for characterizing the diffusion coefficient of Rn so as to determine whether the process has reached steady-state or not. Zeolite 4A was selected as testing porous material. The NLSM method was shown by the results to be able to agree well with the results by the conventional method while the process had already reached steady-state. The results show that the apparent diffusion coefficient of zeolite 4A of porosity 0.62 is 8.31¿10-6 m22 s-1 from NLSM calculation. The Rn adsorption coefficient for this porous matrix is about 6.9¿10-4 m3 kg-1 derived on the basis of linear adsorption isotherm assumption. The method could be applied toward other porous materials, i.e., soils and building materials that are the primary sources of indoor radon. ¿American Geophysical Union 1993