Polymer tantalum capacitors are emerging as an excellent choice for circuit designs that requires stable capacitance, long life, high reliability and smaller size. Conductivity stability of PEDOT materials as well as polymer tantalum capacitors with PEDOT cathodes were investigated in this study for stability under high humidity high temperature conditions. PEDOT films and cathodes formed from insitu polymerization (PEDOT: TOS) and pre-polymerized dispersion (PEDOT: PSSA) were compared under 85°C/85%RH environment. Electrical measurements along with Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), UV Spectroscopy, Thermo-mechanical Analysis (TMA) and Thermal Imaging were used to characterize the samples. Resistance of the films formed from insitu polymerization increased significantly on exposure to 85C/85%RH compared to the films formed from pre-polymerized dispersion. However, the trend was reversed in the capacitors where cathodes formed by insitu polymerization was more stable than the cathodes from PEDOT: PSSA. Our studies suggest that delamination caused by the thermomechanical and voltage induced stress generated in these capacitors is primarily responsible for resistance degradation in polymer capacitors. These new insights led to the development of a new generation of PEDOT:PSSA polymer systems which significantly enhanced the reliability of polymer capacitors in harsh environments.