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The clear substrate-dependent growth and delayed film continuity are essential challenges of Ru atomic layer deposition (ALD) demanding adequate and versatile approaches for their study. Here, we report on the application of in situ Angle Resolved X-ray Phototelectron Spectroscopy (ARXPS) for investigation of initial and steady-state ALD growth of Ru using Ru(EtCp) 2 and O 2 as precursors. Using ARXPS surface analysis technique we determine such parameters of Ru ALD initial growth as incubation period, fractional coverage and the thickness of islands/film depending on the substrate chemical state, governed by the presence/absence of NH 3 /Ar plasma pretreatment. It was demonstrated that NH 3 /Ar plasma pretreatment allows to obtain the lowest incubation period (∼7 ALD cycles) resulting in a continuous ultrathin (∼20 A) and smooth Ru films after 70 ALD cycles. In situ XPS at UHV was used at steady state Ru growth for analysis of half-cycle reactions that revealed formation of RuO x (x ≈ 2) layer with thickness of ∼8 A after O 2 pulse (first half-cycle). It was also shown that oxygen of RuO x layer combusts Ru(EtCp) 2 ligands in the second half-cycle reaction and the observed Ru growth of ∼0.34 A per cycle is in a good agreement with the amount of oxygen in the RuOx layer.