This paper deals with the partial-discharge (PD) behaviour of some insulating fluids used in transformers. The PD are generated on a needle-plate electrode configuration. At first the influence of different parameters such as needle-tip radius and gap distance on the PD inception voltage and other relevant PD characteristics of mineral-based transformer oil are discussed. It follows a comparison of the PD behaviour of a mineral-based transformer oil and a synthetic fluid, used as insulating liquid in transformers. Based on the results it is shown that the initiation and the propagation of PD phenomena depends on different parameters.