Impregnated-paper capacitors can fail in service from a variety of causes depending upon manufacturing and operating conditions. Under normal working conditions, the usual slow deterioration which ultimately leads to failure may be of an electrochemical nature. Factors influencing this type of deterioration are dealt with in some detail. Adverse factors are impurities, such as rosin and bitumen, which dissociate in the dielectric and accelerate the electrochemical processes, or the presence in the dielectric of moisture arising from poor impregnation or imperfect sealing. The effects of contamination are greater when polar impregnants are used, e.g. chlorinated hydrocarbons.Electrochemical deterioration of chlorinated dielectrics can be greatly reduced by the addition of “stabilizers.” A theory is advanced to explain the action of these stabilizers, and experimental evidence in support of the theory is presented.