An electrical discharge that occurs at an oil/pressboard interface or pressboard surface is called surface partial discharge. The surface discharge activity can cause irreversible damage to the insulation due to its thermal and chemical actions. It is visualized as white marks or tree-like structures on the surface of pressboard. In this paper, surface discharge characteristics of synthetic ester/cellulose-board have been investigated to study the influence of moisture, copper particles, and gas bubbles. The partial discharge (PD) experiments were carried out using a needle-plane electrode. The measurements of partial discharge inception voltage (PDIV), apparent charge and phase-resolved partial discharge patterns were acquired. In order to compare the effect of moisture over two independent insulations, separate PD measurements of the moisture-in-oil and the moisture-in-pressboard were conducted. The experimental results show that the moisture-in-oil samples have low PDIV values and more discharge pulses over the moisture-in-pressboard samples. Further, in the case of metallic particles, the PDIV and the number of repetitive pulses were increased when copper particles on the pressboard surface. The measurements of PD patterns of bubble samples are in good agreement with available literature on field measurement of 132/11 kV rated transformer. The findings of this study will assist in understanding the behavior of surface PDs under various conditions and would be of interest to asset managers considering the effects of such conditions on the transformer insulation diagnostics. © 1994-2012 IEEE.