A highly conjugated dimer of nickel(II) octaethylporphyrin linked by a butadiyne bridge on a meso position has been used for the deposition of langmuir-Blodgett (LB) films in mixture with arachidic acid. Similar porphyrin derivatives have never been used before as the active layer in gas sensors. Linearly polarized light UV-vis absorption spectra do not show evidence of in-plane or out-of-plane anisotropy of the orientation of molecules in the LB films. Structural and morphological analysis were also carried out by transmission electron microscopy techniques. A high degree of crystallinity in the observed films is apparent from both selected area electron diffraction patterns and high-resolution images. Unfortunately, the films degrade rapidly on exposure to the electron beam. In addition, an ammonia sensor based on this dimer has been investigated. Eighty-four-layer LB films exhibit satisfactory sensing characteristics for NH3 with a low working temperature (50 degrees C) and a minimum detection limit of 30 ppm.