Two asymmetrically substituted porphyrins (5,15-bis(4-nitrophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)-phenyl)-21H,23H-porphine, CAH3; 5,15-bis(4-aminophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)21H,23H-porphine, CAH4) have been used in this work. Good Langmuir monolayers of these compounds have been prepared on water and transferred successfully to glass slides at much faster than conventional deposition rates (500 mm min(-1)). The monolayer behavior has been investigated by measuring surface pressure-area isotherms as well as by Brewster angle microscopy (BAM) and reflection spectroscopy. For both porphyrins, the intermolecular pi-pi interaction is sufficiently strong that significant preaggregation occurs prior to compression of the film. However, as the monolayers are compressed, changes in the molecular association and orientation are observed for CAH4 while no molecular reorganization is appreciated for CAH3. The different molecular packing of the Langmuir monolayers is maintained during the transfer. The UV-vis spectra of CAH3 and CAH4 solutions, respectively, were found to be sensitive to NO2 gas. However, only the Langmuir-Blodgett (LB) assemblies of CAH4 showed a response in the presence of the toxic gas. This different behavior has been explained in terms of the different molecular organization of the two porphyrins.