The failure mechanism of solid tantalum and aluminium capacitors have been investigated using a combination of electrical measurements and electron microscopy. The capacitor dielectric was examined before and after life testing and changes correlated with electrical measurements.The basic mechanism of failure of solid tantalum capacitors is found to be field crystallisation of the essentially amorphous dielectric oxide. The growth of higher conductivity crystalline oxide during operation of the capacitors causes an increase in leakage current and may result in catastrophic failure. The effect of field crystallisation can be minimised by using high purity tantalum to reduce the number of crystallisation nucleation sites. Since crystalline growth is primarily dependant on applied voltage, high voltage capacitors are much more susceptible to failure than low voltage units.There appears to be no long term failure mechanism in solid aluminium capacitors. However, a particular problem with these units is that they are difficult to make. This is because the anodic layer is chemically less stable in the case of aluminium than in the case of tantalum. The attack is initiated during the deposition of manganese oxide by pyrolysis from manganese nitrate solution and developed by the reform process. Solid aluminium capacitors often have a lower capacitance and higher initial leakage current than comparable solid tantalum units; however, the leakage current decreases on life tests and their reliability is high.