[en] The gas complexation reactions between LnCl₃ and Al₂Cl₆ to yield molecular species of the form LnCl₃ (Al₂Cl₆)/sub x/ have been studied for a number of years. The spectrophotometric technique has been used to deduce the thermodynamic properties of the molecular species. We have studied the HoCl₃ (Al₂Cl₆)/sub x/ system from 600 to 900⁰K and at Al₂Cl₆ pressures between 1 and 7 atm. The Ho³⁺ hypersensitive transition between ⁵I₈ and ⁵G₆ at 456 nm has been used to measure the complex vapor densities as a function of pressure and temperature. A temperature-dependence of the optical absorption spectrum was observed in samples with a constant Ho³⁺ ion density. These data cannot be understood in terms of the generally used treatment of molar absorptivity, but requires the introduction of an effective oscillator strength. A temperature-dependent oscillator effect is also observed and this is interpeted as evidence for the presence of multiple species with differing oscillator strengths. The consequence of these observations for the determination of equilibrium constants for the different species will be discussed