This paper presents the interpretation of both time and frequency domain dielectric response measurements namely Polarization and Depolarization Current (PDC) measurement and Frequency Domain Spectroscopy (FDS) for evaluating transformer insulation under different thermal conditions. An intelligent algorithm developed by the authors of this paper is applied to interpret PDC measurements of three model transformers and evaluate the insulation conditions of these transformers under stable and transient temperatures. The effects of temperature variations and transients on FDS measurements of transformer insulation are also investigated in this paper.