A series of gas-oil hydrotreating experiments with different durations ranging from 0.5 to 24 h were conducted with a view to gain a clear understanding of the nature of the coke that deposits on the catalyst surface during the early period of the operation and its role in catalyst deactivation. The results showed that a rapid coke buildup on the catalyst surface starts as soon as the feed is introduced, reaches as high as 10 wt% within 3 h and then tends to level off or reaches an equilibrium value within 24 h. The initial coke affected the HDS and HDN functions of the catalyst differently. The drop in catalyst surface area was particularly high (40%) during the first 3 h of operation. Pore size distribution analysis indicated that the coke which formed during the early hours of the run was deposited predominantly in the narrow pores (<50 Angstrom) of the catalyst. Temperature programmed oxidation (TPO) analysis of the catalysts coked for different durations showed significant changes in coke characteristics with increasing process time. Part of the initial coke was found to be strongly bound to the catalyst requiring a higher temperature for its removal by oxidation.