Two kinds of Er-doped lithium phosphate glass have been chemically strengthened by ion exchange in a mixed binary melt of KNO3 and NaNO3 at temperature far below glass Tg. Ion depth profile has been determined with WDS technique. The effect of acid wash on surface morphology, hardness and fracture temperature of glass was investigated. It was found that acid wash following ion exchange could improve the surface morphology and strength of glass. Nevertheless, WDS analysis showed that the acid wash could cause the ion diffusion from glass surface to acid wash liquid, i.e. the back diffusion of K+ and Na+. The results revealed that the back diffusion of K+ was severer than that of Na+ during acid wash and the bond of K+ to glass network was weak.