Microstructural properties of ultrathin (1-10 nm) tetrahedral amorphous carbon (ta-C) films are investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray Photoelectron Spectroscopy, Raman spectroscopy and Atomic Force Microscopy (AFM). The CK-edge NEXAFS spectra of 1 nm ta-C films provided evidence of surface defects (C - H bonds) which rapidly diminish with increasing film thickness. A critical thickness for stabilization of largely sp 3 matrix structure distorted by sp 2 sites is observed via the change of ?*C*C peak behavior. Meanwhile, an increase in the film thickness promotes an enhancement in sp 3 content, the film roughness remains nearly constant as probed by spectroscopic techniques and AFM, respectively. The effect of thickness on local bonding states of ultrathin ta-C films proves to be the limiting factor for their potential use in magnetic and optical storage devices. © 2011 Elsevier B.V. All rights reserved.