This article presents the results of the studies concerning initiation and breakdown in biodegradable dielectric liquids and two types of mineral oils under the lightning impulse voltage of negative polarity. The electrode system, in which a high voltage electrode as well as a grounded one were insulated with solid insulation, are adopted for the present study. Within the analysis made, the inception voltage and breakdown voltage as well as time to breakdown and intensity of light emitted by developing discharges are compared. The results show that under present experimental conditions (insulated electrodes and negative lightning impulse voltage), the behaviour of ester liquids is inferior. Both the inception voltage as well as the lightning impulse breakdown voltage are lower in the case of esters as compared to the mineral oils.