Hydrogen permeation through Zr-1%Nb alloy was studied at the temperature below the temperature of a-b transition. Analysis of the transient permeation curves from a closed volume in a surface limited regime allowed to determine total and mobile hydrogen concentrations. At the mobile hydrogen concentration of 4.3 at% a part of the absorbed hydrogen is cut out of permeation process. Increase of the mobile hydrogen concentration in a-phase of Zre1%Nb alloy is ceasing at the concentration of (5.5 ± 0.3) at%, which is the maximum possible concentration of the mobile hydrogen in a-phase of the studied alloy. From this moment on all absorbed hydrogen is spent on hydride formation. The obtained results are compared with those obtained by means of traditional techniques for terminal solid solubility determination.