We report a quantum mechanical study on the electrostatic interactions found in poly(L-lysine)(.)alkyl sulfate complexes, which are materials with interesting technological properties able to adopt self-assembled supramolecular structures. For this purpose, 19 reduced complexes were considered in the gas phase, chloroforrn solution, and aqueous solution. Calculations in the gas phase were carried out up to the MP2/6-311++G(d,p) level, while the influence of the bulk solvent was investigated at the HF/6-311++G(d,p) level using the polarizable continuum model. The electrostatic interaction characteristics of poly(L-lysine)(.)alkyl sulfate complexes have been compared with those of alkyltrimethylammonium(.)poly(alpha,L-glutamate) complexes, a related family of materials previously investigated.