S passivation of a HfO2 film on an InP substrate is demonstrated using annealing in H2S ambient either before or after ALD of the HfO2 film. We examined the resulting distribution and chemical state of the incorporated S both in the HfO2 film and at its interface with the InP substrate using secondary ion mass spectroscopy and X-ray absorption spectroscopy. Annealing in H2S ambient before ALD of HfO2 resulted in accumulation of S at the interface in the sulfide (S2-) phase (due to lack of oxygen), which effectively suppressed interfacial reactions during ALD and passivated interfacial defect states. On the other hand, annealing in H2S ambient after ALD of the HfO2 film induced a sulfate (S6+) phase in the film due to the abundant oxygen in the film, as well as a sulfide (S2-) phase at the interface. This improved the charge trapping behavior by decreasing the bulk defect density in the HfO2 film.