Three types of experiments were used to study the surface chemistry of silicon in alkaline solution: minority carrier injection from a p-n junction electrode, in-situ photoluminescence, and electron transfer to a redox system in solution. The results lead to the conclusion that the surface chemistry and electrochemistry are determined to a large extent by an activated intermediate of the chemical etching reaction of silicon with water. This novel coupling of chemical and electrochemical steps can account for some unusual features of the system, such as a mechanism for anodic oxidation and passivation based on electron injection and the strong influence of a weak oxidizing agent on the surface morphology of chemically etched silicon.