The monolayer and deposition behaviours of 1,4,8,11,15,18-hexa-alkyl-22,25-bis (carboxyethyl)phthalocyanines and 1,4,8,11,15,18,22,25-octa-alkylphthalocyanines have been compared. Compounds in the former series show better monolayer behaviour at the air-water interface than those in the latter series, which form rigid films that cannot be deposited onto substrates. Examples of the first series are excellent for fabrication as Langmuir-Blodgett (LB) films, giving evenly deposited well-ordered bilayer assemblies by Y-type deposition. The effect of temperature on an LB film of one of the compounds has been monitored using polarized absorption spectroscopy and low-angle X-ray diffraction. Up to 50°C the film undergoes some initial reorganization of what is presumed to be a metastable state formed during initial deposition. The molecular structure undergoes a more substantial transition at 127.5°C over a 0.5°C temperature range to a modified film in which adjacent molecules lose translational non-equivalence. Furthermore, there is a reduction in the bilayer thickness from 41 to 36 Å. This process is irreversible as judged by UV-visible spectroscopy.