Langmuir-Blodgett films of a 1:4 mixture of meso,meso'-buta-1,3-diyne-bridged N(II) octaethylporphyrin dimer and arachidic acid have been deposited onto various substrates. The isotherm at the air/water interface and absorbance measurements using polarised light are consistent with a planar statistical orientation of the porphyrin dimers which are probably prone onto the arachidic acid tails. The presence in the dimer of the acetylenic linking group allows the formation of a large and highly conjugated molecular structure; this has suggested the use of the films in gas sensing apparatus. Electrical characterisation in controlled atmosphere and with the simultaneous presence of interfering gaseous species, such as C2H5OH, NH3, and CO, suggest that this porphyrin dimer is a promising selective NO gas sensing material.