Monolayers of alkyl side chain substituted preformed polyimides (BACBP/BPDA) exhibit behaviour different from that of other ＇hairy-rod＇ polymers which were studied previously. This behaviour is thought to arise from the extreme rigidity of the backbone as well as the particular way in which the alkyl side chains extend from the biphenyl unit. These soluble polyimides can form metastable monolayers for which the pressure-area isotherms vary markedly with side chain length. This side chain length dependence suggests that the side chains can not readily project away from the water surface, but rather occupy considerable space at the interface at lower temperatures. For the polyimide with octadecyl side chains, a sharp reduction in zero pressure area occurs between 20 and 24 degrees C, suggesting an important change in side chain mobility, while the polyimide with heptyl side chains shows no strong dependence of zero pressure area on temperature. These materials are promising for creating robust molecularly thin layers.