Factors determining etching and passivation of n-type InP in H2SO4 and HCl solution and the corresponding surface chemistry are considered. Passivation is favoured by higher light intensity and lower proton and Cl− ion concentration. Ex-situ surface analysis shows the passive (bi)layer to consist mainly of oxide-based In3+ and P5+ components: InPO4 and In(PO3)3. Hydrodynamics is found to play a decisive and surprizing role in determining the kinetics of the surface reactions. Oxygen-bridge formation between surface In and P atoms, as a result of deprotonation of a P-OH reaction intermediate, is considered to be important in determining competition between the two reaction paths: etching and passivation. These results are compared with markedly contrasting results for n-type GaAs under similar experimental conditions.