Aging of epoxy based composites used as insulators in the industry are investigated. The analysis of a composite aged for 20 years in actual service conditions points out chemical degradations, a structural recovery, a tendency for particle debonding from the polymer matrix and with respect to the unaged sample, and no significant changes in the permittivity values measured at 50 degrees C. To understand this latter result, a purely physical aging was carried out on fresh composites with similar formulation as the insulator aged 20 years in service. A detailed analysis of the changes in properties during physical aging is proposed. It is concluded that the constant epsilon' value at 50 degrees C found for the 20 years aged composite results from two opposite phenomena: an increase in the relative dipole amount due to structural recovery and/or the presence of polar species at the polymer/filler interface that both increase epsilon' and the loss of polar species in the bulk that decreases the epsilon'. (c) 2006 Wiley Periodicals, Inc.