Past work has shown that surface‐active environments can influence the hardness of such nonmetallic solids as silver chloride, magnesium oxide, alumina, quartz, and soda‐lime glass. Specifically, a maximum in hardness can be produced when the &zgr; potential of these solids is zero. This ’’&zgr; correlation’’ has been taken to imply that surface charge can markedly influence near‐surface hardness and, in particular, that solids are hardest when their surface charge is zero. To test this implied relationship between surface charge and hardness in a more direct fashion, the hardness of the (0001) surface of ZnO has been measured as a function of surface charge controlled using an electrolytic cell. The results of this study demonstrate that for ZnO a maximum in hardness is produced, but apparently not when the surface charge is zero. Rather, the hardness maximum is observed when the ZnO surface is positively charged (modest downward band bending). Moreover, previous suggestions attempting to explain the ’’&zgr; correlation’’ are shown to be inapplicable to the present results on ZnO, and an alternative mechanism involving charge exchange between donor levels and the conduction band is suggested.