The paper studies the dependence of the electrical resistance and fluorescence spectra of mixtures of dielectric liquids for power transformers on the relative concentration of the components of the mixture. The electrical resistance at direct current was estimated using three different formulas. It is shown that for the mixtures of “GK” brand petroleum oil and dielectric fluid MIDEL eN none of the formulas allows obtaining an acceptable accuracy of estimation. The L. K. H. van Beek formula most accurately describes the character of the change in the resistance of the mixture when the concentration of the components of the source liquids changes, and for artificially aged mixtures, none of the formulas can describe the non-monotonic character of the change in resistance. We found that the fluorescence spectrum is approximately determined by the sum of the fluorescence spectra of each of the components with coefficients equal to the relative concentration of the components. A lower susceptibility to thermal aging of the mixture of petroleum oil and dielectric fluid MIDEL eN at a certain concentration of components has been confirmed.