An analysis is presented of the thermodynamics of lattice defects and H or D in rutile. It is shown that the solubility of these imperfections depends strongly on electron Fermi level, which in turn depends on the concentration of the imperfections. A method is presented for using hydrogen solubility to determine conduction band parameters and the energies and densities of electron traps in the crystal. The conditions for obtaining large or small concentrations of H,D or lattice defects are discussed. The use of the method presented depends on an accurate calibration of the oscillator strengths of the H‐D ir absorption in rutile. Such a calibration is presented in the following paper.