Ionic liquid crystalline polymers were obtained from linear poly(ethyleneimine) (PEI) substituted with 6-(4-methoxy-4′-biphenyloxy)hexyl bromide or 12-(4-methoxy-4′-biphenylyloxy)dodecyl bromide, and quaternised with dimethyl sulfate or hydrobromic acid. Their thermotropic polymorphism was studied by means of differential scanning calorimetry (DSC), polarising optical microscopy and X-ray diffraction. A smectic A mesophase was identified and described from a structural point of view with the help of computer modelling. The completely alkylated and quaternised PEI derivative was shown to form a single layered smectic A structure with pendant groups all on the same side of the polymer backbone in a double row ribbon-like configuration. The ribbon-like polymer chains are packed side by side with the polymer backbone parallel to the smectic layers and the mesogenic groups perpendicular to them. For partially alkylated or quaternised PEI derivatives, the layer spacing in the smectic A mesophase is generally different from that of the corresponding molecular length calculated for fully functionalised polymers.