In situ real-time scanning tunneling microscopy is used to study Si etching in alkaline solutions modified by addition of Triton, a commonly used nonionic surfactant. On Si(111), time sequences of images, with the resolution of-atomic steps, show that the rate of the nucleation of etch pits is decreased on terraces which reduces the etch rate. Results are interpreted in term of formation of a self-assembled micellar layer of Triton molecules whose disorder in the vicinity of atomic scale defects improves surface order compared to etching in NaOH. This layer being not bound to the surface, the (1 x 1)-H-terminated Si(111) surface is always imaged at high resolution. Potential applications of Triton solutions to the preparation of flat surfaces of Si(111) and (100) are discussed.