Two-level system (TLS) defects in dielectrics are known to limit the performance of electronic devices. We study TLS using millikelvin microwave (6.4GHz) loss measurements of three atomic layer deposited (ALD) oxide films-crystalline BeO (c-BeO), amorphous Al2O3 (a-Al2O3), and amorphous LaAlO3 (a-LaAlO3)-and interpret them with room temperature characterization measurements. We find that the bulk loss tangent in the crystalline film is 6 times higher than in the amorphous films. In addition, its power saturation agrees with an amorphous distribution of TLS. Secondary ion mass spectrometry (SIMS) impurity analysis of the c-BeO film showed excess surface carbon (C) impurities and a uniform hydrogen (H) impurity distribution, which coupled with the analysis of loss tangent strongly suggests H limited loss. Impurity analysis of the amorphous films reveals that they have excess H impurities at the ambient-exposed surface, and we extract the associated H-based surface loss tangent. We compare two a-Al2O3 films with drastically different C impurity concentrations and similar H impurity concentrations and conclude that H rather than C is the likely source of loss in the amorphous films and we find the loss per H concentration in a-Al2O3 to be K-H = 3 x 10(-24) cm(3). (C) 2013 AIP Publishing LLC.