A method for measuring flow porosity with values from 35 to 100% of the open porosity measured is described. The accuracy of electrical conductivity measurement to obtain net diffusibilities was verified for media containing at least 75% of pore volume with radii greater than five mean free paths. It is pointed out that long dead‐end pores feeding into larger diameter flow pores can contribute to the effective diffusion coefficient inside porous media, and methods of estimating their length and maximum possible contribution to diffusion are described.