The film-forming behaviour of octadecyltrimethoxysilane (TMS) and octadecyltriethoxysilane (TES) was investigated. At sufficiently low subphase temperatures (less than 25 degrees C for TMS and less than 15 degrees C for TES), the surface pressure isotherms show a temperature-dependent transition from a liquid-expanded to a liquid-condensed phase. On further compression, a second transition is observed to an even more condensed phase in which it is believed that the alkoxy headgroups assume a staggered arrangement so as to minimize the area per molecule. Monolayers of TMS deposited from this phase are uniform and highly ordered. Fourier transform infrared (FTIR) studies carried out over a range of incident angles suggest a vertical alignment of the aliphatic chains in the monolayer. Surface potential measurements on deposited monolayers indicate the possibility of a long-term chemical reaction between the alkoxy headgroup and the underlying aluminium substrate.