Polyelectrolyte complexes of high-molecular-weight poly(ethylene imine) (PEI) and n-alkyl carboxylic acids were prepared. They were investigated with the use of differential thermal analysis, small- and wide-angle X-ray scattering techniques, and fluorescence spectroscopy. The chain lengths of the carboxylic acids varied from decanoic acid (C10) to hexacosanoic acid (C26). Order−order transitions from smectic C to smectic A structures were found, whose enthalpies increase with 3 kJ/mol per CH2 group. The transitions result from the melting of n-alkyl side chains, which are packed into a two-dimensional hexagonal lattice. The transition temperatures, Tm, increased from −2 to 83 °C when the chain lengths were increased from C12 to C26. Thomson's rule was used to arrive at a quantitative description of the dependency of Tm on the chain length. Pyrene was incorporated into the complexes, and the apparent dielectric constant of its surroundings was determined to decrease stepwise with the increasing length of the side chains.