Abstract This work evaluates the passivation efficacy of thermal atomic layer deposited (ALD) Al 2 O 3 dielectric layer on self-catalyzed GaAs 1- x Sb x nanowires (NWs) grown using molecular beam epitaxy. A detailed assessment of surface chemical composition and optical properties of Al 2 O 3 passivated NWs with and without prior sulfur treatment were studied and compared to as-grown samples using x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature photoluminescence (PL) spectroscopy. The XPS measurements reveal that prior sulfur treatment followed by Al 2 O 3 ALD deposition abates III–V native oxides from the NW surface. However, the degradation in 4K-PL intensity by an order of magnitude observed for NWs with Al 2 O 3 shell layer compared to the as-grown NWs, irrespective of prior sulfur treatment, suggests the formation of defect states at the NW/dielectric interface contributing to non-radiative recombination centers. This is corroborated by the Raman spectral broadening of LO and TO Raman modes, increased background scattering, and redshift observed for Al 2 O 3 deposited NWs relative to the as-grown. Thus, our work seems to indicate the unsuitability of ALD deposited Al 2 O 3 as a passivation layer for GaAsSb NWs.