Ultrathin films of polythiophene derivatives spread on water were studied by means of synchrotron radiation, using grazing incidence diffraction and (specular) reflectometry to obtain the molecular orientation in the films. The semierystalline films were anisotropic, showing a strong tendency of orienting the crystalline a-axis perpendicularly to the water subphase. The crystalline domains extend essentially through the entire sample thickness, found to be 10-15 nm. A large expansion of the unit cell a-parameter was seen upon doping the films in situ. The reflectometry data were well-fitted by a model with a sinusoidal density variation being damped toward the water subphase. This indicates that the crystalline order was most developed at the polymer-air interface and deteriorated down toward the water, possibly due to the hydrophobicity of the alkyl side chains.