The anodic dissolution and passivation of Si in aqueous hydrazine have been studied, with special emphasis on the effects of hydrodynamic conditions, by using a submerged impinging jet cell. Experiments, performed in the temperature range 20-50°C, show an anomalous current decrease on increasing mass-transfer rate, more pronounced at lower temperatures and observed both in the presence and in the absence of dissolved oxygen. It is shown that data may be interpreted by a mechanism similar to that previously proposed for Si dissolution in NaOH. As a result of electrode processes and of the electroneutrality constraints in a binary electrolyte, the equal surface concentrations of the two ionic species, and increase above the bulk value; one of these species, most probably has a catalytic action on the dissolution steps and its local buildup promotes a larger current; increased mass transport, by reducing the surface concentration excess, reduces the current. The model explains the main experimental data, including the marked decrease of the mass-transport effect upon addition to the solution of significant concentrations of either NaOH or NaCl.