Foreign gases contaminating the inhaled air are absorbed in the respiratory tract by diffusion of their molecules to the surfaces of the airways. If all contaminant molecules which struck the surfaces were eliminated, no gas would penetrate beyond the dead space of the lungs and none would enter the alveolated region. Elimination at the wall depends either on the solubility of the foreign molecules or on chemical fixation. Some gases, in high enough concentration, can saturate the surfaces of the dead space airways which stops diffusion so that penetration to the alveoli takes place. Formulae are presented for calculating the saturation vapour concentrations and diffusion coefficients, when experimental values are not available, and a number of values are listed. Large molecules diffuse exactly like single particles. Finally, the contrasting behaviour of seven different gases, when inhaled, is accounted for in terms of the basic principles described in the preceding parts of the paper.