In this work, composite microelectrodes from poly(3,4-ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNT) are characterized as electrochemical sensing material for neurotransmitters. Dopamine can be detected using square wave voltammetry at these microelectrodes. The CNTs improve the sensitivity by a factor of two. In addition, the selectivity towards dopamine in the presence of ascorbic acid and uric acid was examined. While both electrodes, PEDOT and PEDOT-CNT are able to detect all measured concentrations of dopamine in the presence of uric acid, small concentrations of dopamine and ascorbic acid are only distinguishable at PEDOT-CNT electrodes. Changing the pH has a strong influence on the selectivity. Moreover, it is possible to detect concentrations as low as 1 µM dopamine in complex cell culture medium. Finally, other catecholamines like serotonin, epinephrine, norepinephrine and L-dopa are also electrochemically detectable at PEDOT-CNT microelectrodes.