Metal@semiconductor core−shell nanoparticles (NPs) are widely used in photocatalysts, sensors, and optical applications owing to their unique metal−semiconductor interface and the integration of the properties from both core and shell materials. Although many efforts have been made toward the precise synthesis of Au@Cu 2 O core−shell structures, the chemical stability of Au@Cu 2 O aqueous suspensions, which is of great significance in many related applications, is not mentioned in any published research. Herein we report the synthesis of Au@Cu 2 O core−shell NPs with small shell thickness from 2 to 40 nm through a wet-chemistry method. The UV−vis absorption properties are found to be tunable with Cu 2 O thickness in the range of 2−40 nm. Furthermore, the chemical stability of Au@Cu 2 O core−shell nanoparticle suspensions in water/ethanol mixed solvents is investigated. It is found that water/ethanol mixed solvents with a larger amount of water are more likely to deteriorate the stability of Au@Cu 2 O NPs by oxidizing Cu 2 O to CuO. The results from this work may provide useful information for the preparation of metal@Cu 2 O water-based suspensions that are expected to be used for SERS, photocatalyst, or photothermal applications.