The structure of the smectic B phase formed by perfluorodecylalkanes has been analyzed through the synthesis of three molecules and a careful investigation of their mesophases. Molecules with C-8, C-10 and a branched C-11 alkyl chain were considered. The structure factor analysis of 001 reflexions is consistent with the picture of alternating electron rich and electron poor sublayers. A structural model with segregated perfluoroalkyl chains in the middle of the layer and disordered alkyl chains on each sides is proposed. This model is discussed in terms of space filling arguments: chain lengths, cross section compatibility in the layer and compactness. Infrared dichroic ratios of the CH2 and CF2 stretching modes confirm that perfluoroalkyl subunits are perpendicular to the S-B layer while alkyl ones, in a quasi molten state, fill the remaining layer space.