The photoabsorption spectra of calcium-doped argon clusters CaArn are investigated at thermal equilibrium using a variety of theoretical and numerical tools. The influence of temperature on the absorption spectra is estimated using the quantum superposition method for a variety of cluster sizes in the range 6less than or equal tonless than or equal to146. At the harmonic level of approximation, the absorption intensity is calculated through an extension of the Gaussian theory by Wadi and Pollak [J. Chem. Phys. 110, 11890 (1999)]. This theory is tested on simple, few-atom systems in both the classical and quantum regimes for which highly accurate Monte Carlo data can be obtained. By incorporating quantum anharmonic corrections to the partition functions and respective weights of the isomers, we show that the superposition method can correctly describe the finite-temperature spectroscopic properties of CaArn systems. The use of the absorption spectrum as a possible probe of isomerization or phase changes in the argon cluster is discussed at the light of finite-size effects. (C) 2003 American Institute of Physics.