The permeation properties of molybdenum coatings deposited as hydrogen diffusion barriers are studied. Absorption-desorption experiments with molybdenum-coated nickel substrates (previously published) are quantitatively interpreted using a non-steady-state analytical solution, developed in detail in a companion paper. Given the known solubility and diffusion coefficient of hydrogen in nickel, the effective permeation constant of the molybdenum coating is measured to be (8.4+/-2.5)X10(-16) and (37+/-13)X10(-16) m(2) s(-1) at at(-1) at 350 and 450 degrees C, respectively. Surface and interface effects are evident. While performing rather well as hydrogen diffusion barriers, the coatings exhibit about 9 (at 450 degrees C) or 17 (at 350 degrees C) times less protection than expected from literature diffusion data for bulk molybdenum. The relatively low efficacy of the molybdenum coating is attributed to material microdamage resulting from high internal stress.