The application of III–V semiconductors, including GaAs, in various optoelectronic devices has attracted considerable attention due to their high electron mobility. The presence of native oxide on GaAs surfaces causes a high surface density of states that can be removed by passivation techniques. One of the most common passivation approaches is the ammonium sulfide ((NH4)2Sx) treatment; however, the GaAs surface tends to re-oxidize after passivation in a short time. This study presents the long-term passivation of GaAs surfaces induced by the use of a saturated ammonium sulfide solution under optimum condition. The X-ray photoelectron spectroscopy (XPS) study was conducted to prove our aim. XPS results demonstrate the removal of the native oxide layer from the GaAs surface immediately after passivation and the absence of surface re-oxidation for several days after passivation and air exposure. Therefore this approach can be used as an effective way to achieve stable passivated surface of GaAs.