Photoelectron emission spectroscopy in air (PESA) has been used to investigate titanium dioxide (TiO2) deposited by atomic layer deposition (ALD). A procedure has been developed to unambiguously determine the photoemission threshold energy (also referred to as the "ionization potential") of TiO2 thin films, avoiding inherent artifacts due to photoelectron emission from the substrate, which supplies misleading results. This has been achieved using PESA measurements performed as a function of TiO2 film thickness on two substrates with different work functions. We find that proper measurements of the photoemission threshold energy (including work function and ionization potential) of thin films by PESA require the use of films much thicker than their electron attenuation length (EAL). A photoemission threshold energy of 5.0 +/- 0.2 eV is obtained for TiO2 and has been attributed to a trap level due to oxygen vacancies, which lie within the band gap of the TiO2. The analysis of the photoemission decay with film thickness also provides a method for determining a "practical" (or effective) EAL at excitation energy slightly above the photoemission threshold energy of the material. We extract an EAL for the deposited TiO2 of 0.65 +/- 0.02 nm (at 0.5 eV). The procedure can also be adopted for determining the thickness of extremely thin films, provided their thickness is smaller than their EAL. (C) 2017 Elsevier B.V. All rights reserved.