The molecular structure and composition of carbonaceous deposits commonly known as coke deposited inside the pores and on the sill-face of aged hydroprocessing (hydrotreating and hydrocracking) catalysts in value upgradation of vacuum gas oils were investigated by various analytical methods, such as NMR, HPLC and TGA. The soft (dichloromethane extractable) and hard (solvent insoluble) coke deposits from two spent hydrotreating catalysts (CoMo/gamma-Al2O3 and NiMo/gamma-Al2O3) and one spent commercial hydrocracking catalyst with carbon contents ranging from 5 to 10% were characterized. HPLC and high resolution liquid state H-1 and C-13 NMR methods were used for the characterization of the soft coke whereas hard insoluble coke deposits were characterized by high resolution solid-state C-13 NMR and TGA. The results indicate that the composition of soft coke is mostly alkylated mono-, di-aromatics and fewer amounts of polyaromatics. The composition of the hard coke in spent hydrotreating catalysts is relatively high in aliphatic content with high H/C ratio obtained in the present study unlike the highly aromatic coke obtained with residue hydrotreating as reported in the literature. However, the insoluble coke deposits in spent hydrocracking catalysts are highly aromatic in nature (aromaticity, f(a) > 0.95). (C) 2004 Elsevier B.V. All rights reserved.