Two monolayers of didecanoyllecithin at the air-water interface have been studied using molecular dynamics simulations. The model system consisted of two monolayers of 42 lecithin molecules each separated by a roughly 4 nm thick slab of SPC water. The area per lecithin molecule was 0.78 nm(2) corresponding to a fluid phase. Three different force fields were tested. All models reproduced the experimental surface tension, but the structural and dynamic properties differed significantly between the models. The model with reduced charges on the head groups and Ryckaert-Bellemans dihedral potential in the tails best reproduced experimental data. The standard Gromos potential gave rise to a too much ordered gel-like tail structure. Also the headgroup correlations seem exaggerated with this model and point to the need for a more detailed treatment of the long-range electrostatic interactions between the head groups. For all models the interface between water and lecithin is highly disordered with large but locally varying water penetration. The roughness can partly be explained by capillary waves but must also be an intrinsic property of the monolayer. This is probably due to interactions between the charged groups of the zwitterionic head groups. On average every charge is compensated for by at least one charge from another molecule within a few tenths of a nanometer. Other properties like the electrostatic potential difference across the monolayer, the head group conformation and dynamics, and the structure of the tails have also been analyzed and are discussed.