In recent years, the use of mineral oil and ester fluid mixtures as a possible alternative to mineral oil has attracted the attention of many researchers. However, mixing different insulating liquids comes with many challenges requiring in-depth consideration. One of these challenges is the evaluation of the effect of silver sulfide, forming as a result of corrosive sulfur mixing with the silver layer coating on-load tap changers, on the dielectric properties of the insulating mixture. This paper addresses this issue and reports the results of an experimental investigation to evaluate the effect of adding silver sulfide with different concentration on the dielectric properties of mineral oil-ester fluid mixture. The results show that the addition of silver sulfide to the mixed liquid has reduced the inception voltage due to its semiconductive nature and induced surface charges. Flow electrification studies adopting spinning disc confirms the increase in the streaming current with disc velocity. A negative streaming current was observed in the mixed liquid on its interaction with the pressboard interface. The study also indicates that increased concentration of silver sulfide mixture showed a higher loss tangent and an increase in the streaming current. The physico-chemical properties of the oil through rheological studies observed a marginal change on the addition of silver sulfide to the mixed liquid and it correlates with the electrostatic charging tendency (ECT) of the oil. © 2020 Institute of Electrical Engineers of Japan. All rights reserved.