The development of solid electrolytic tantalum capacitors with MnO2 as counter-electrode has been carried out in order to decrease the equivalent series resistance (ESR). Capacitor samples produced under different pyrolysis conditions have been characterized in terms of equivalent circuit parameters. The Ta/Ta2O5/MnO2 system has also been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray powder diffraction patterns obtained were inconclusive due to the MnO2 complex structure and to the presence of highly intense tantalum peaks that shadow interesting MnO2 diffraction peaks. Electron diffraction TEM results enabled the characterization of the microstructure and furthermore revealed the complex crystalline structure that affects the electrical properties of the semiconductor layer. A relation between the calculated circuit parameters and microstructure of MnO2 is discussed.