Langmuir–Blodgett (LB) films from mixtures of polyaniline (PANI) and the ruthenium complex mer-[RuCl3(dppb)(py)] (dppb=PPh2(CH2)4PPh2; py=pyridine) (Rupy) were used to modify electrodes and to detect dopamine (DA). The electrochemical response has been used as the principle of detection in order to exploit the molecular-level interaction between PANI and Rupy in the LB films. Cyclic voltammograms were performed in a potential range where the Rupy complex is not electroactive and the response is dominated by the interconversion between the oxidation states leucoemeraldine and emeraldine of PANI. In the presence of DA a further redox pair appears at approximately 230 and 0 mV, associated with oxidation/reduction of DA to dopaminequinone. The current increased linearly in the range between 4.0×10−5 and 1.2×10−3 mol/l. The detection limit for DA concentrations for the 21-layer LB film from PANI/Rupy was 4.0×10−5 mol/l, which is sufficient to detect DA in a pharmaceutical product. The modified electrode based on PANI/Rupy LB films is selective to DA, particularly because the oxidation potential for DA is lowered, in comparison to a bare indium-tin-oxide electrode. The presence of ascorbic acid, which is an important interferent for DA, could be detected when its concentration was three times that of DA.