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 pi*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. (C) 2011 Elsevier B.V. All rights reserved.