The optical absorption and thermal conductivity of GaAsPN absorbers are investigated by means of optical absorption spectroscopy and photo-thermal deflection spectroscopy (PDS) for different 100 nm-thick GaAsNP/GaP samples under different growth conditions and various post-growth annealing temperatures. It is first shown that the As content strongly modifies the optical absorption spectrum of the GaAsPN: with a maximum absorption coefficient of 38,000 cm À 1 below the GaP bandgap energy. The optical absorption and thermal conductivities of the samples are then evaluated for various growth and annealing conditions using PDS: the results showing overall agreement with optical absorption spec-troscopy measurements. A significant improvement in optical absorption and thermal conductivity after annealing is demonstrated. The best thermal conductivity measured is equal to 4 W/m K. These results are promising for the development of absorbers in multijunction solar-cell architecture. &