The composition and chemical state of the native oxide of single-crystalline ZnSe and the effect of HCl (18.5%)/H2O wet etching have been studied by time of flight secondary ion mass spectrometry (TOF-SIMS) and x-ray photoelectron spectroscopy (XPS). TOF-SIMS depth profile measurements show that the oxide layer of ZnSe is removed by etching in this solution for one minute, followed by a subsequent one minute rinse in deionized H2O. XPS depth profile measurements of the untreated ZnSe surface shaw that Se oxide only exists at the topmost surface (within the top 10% of the oxide layer). The change of Zn Auger parameter with depth of the untreated ZnSe specimen indicates that the remaining oxygen is chemically associated to Zn. High resolution XPS measurements of the etched ZnSe show no detectable Se oxide at the surface. Meanwhile, the Zn Auger parameter is similar to that of the unetched ZnSe after its oxide layer being removed by Arf sputtering. Both experiments show longer wet etching times result in Zn deficiency and more CI contamination at the sample surface.