Angle- and temperature-dependent surface extended x-ray-absorption fine-structure measurements for the p4g(2×2)NNi(100) structure reveal a strong local vibrational anisotropy of the fivefold-bonded N atoms. At room temperature we find clear evidence for anharmonic vibrations of N relative to its four in-plane Ni neighbors while the vibrations along the fifth bond remain harmonic. This is directly linked to the strong surface reconstruction induced by the chemisorption process. The out-of-plane vibration of the N atom is discussed in terms of an Einstein model. © 1988 The American Physical Society.