A structural characterization of chiral side chain siloxanes with different average degrees of polymerization,DPn, was performed by X-ray diffraction experiments on powder and oriented fibre specimens. Polymers (DPn= 35) and oligomers (DPn= 4) contained the 4,4′-biphenylene unit with either an (S)-2-methylbutoxy(An, Bn)or an (S)-2-chloro-3-methylbutanoyloxy substituent(Cll). The spacer segment connected to the siloxane backbone had a variable number,n, of methylene groups (n= 5, 8, or 11). Independent of the spacer length and the chiral tail nature, the polysiloxanes underwent the same sequence of phases: C-SF1(or SI1)-SC1-SA1-I, whereas in the oligosiloxanes the sequence C-SB1-SA1-I (B11) or C-SF1-SC1-I (B5) occurred. The influence of the structure of the polysiloxanes on the formation of the smectic (tilted or orthogonal) mesophases was elucidated. The rather large number of reflections (three or four) detected in the X-ray patterns at low angles, allowed a drawing of the projection of the electron density profiles along the layer normal,p(z), and deduction of the most physically acceptable electron density profiles from among the numerous possibilities for each smectic phase. The electron density profiles were in agreement with monolayer smectic phases presenting a microphase separation between the siloxane backbones and the side chains, so constraining the polymer backbones within a thin layer.