The invention relates to a method and device for determining a harmonic thickness of an alveolar membrane of a lung, according to which said lung is supplied with an amount of carrier gas carrying a first amount of particles having a size falling within a predetermined range and measuring, after a predetermined period, a second amount of said particles contained in the exhaled gas and then determining, on the basis of the first and second amounts of particles, a parameter indicating a reduction in the amount of particles present in the exhaled gas relative to the supplied gas, and determining, on the basis of said parameter and a sedimentation rate and/or a diffusion coefficient of said particles, a size of a surface on which a sedimentation of the particles and/or a deposition of the particles by diffusion through a layer of alveolar gas has taken place, said harmonic thickness being determined from said size of said surface.
