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, T-m, increased from -2 to 83 degreesC 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 T-m 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.