Atomic force microscopy (AFM) has been used to investigate the structure of arachidic acid Langmuir-Blodgett (LB) films deposited at different surface pressures. The observed molecular arrangements are consistent with a distorted orthorhombic subcell packing containing two molecules per unit cell. For films built-up al high surface deposition pressures (50 mN m(-1)), the unit cell dimensions (a(0) = 0.52 +/- 0.06 nm, b(0) = 0.77 +/- 0.02 nm) are similar to those found for LB layers of fatty acid salts, suggesting that the arachidic acid molecules are aligned with their long axes approximately parallel to the substrate normal. As the deposition pressure is reduced, an increase in both the a(0) and b(0) unit cell dimensions is found. This implies a progressive tilt of the molecules with decreasing deposition pressure.