Spent alumina-based nickel-molybdenum catalysts used for hydroconversion of heavy oil or residuum upgrading are subjected to thermogravimetric analysis (TGA) to determine oil and coke contents simultaneously. TGA or differential TGA curves clearly indicate that oil and carbonaceous materials are driven off at 100-300 degrees C and 300-700 degrees C ranges, respectively. The amounts of oil, coke, and inorganic residue obtained by the single-step TGA procedure are found to be in excellent agreement with those obtained by conventional methods such as Soxhlet extraction for the oil content followed by calcining of the deoiled catalyst in a muffle furnace for the coke and inorganic residue contents. Since the TGA results at heating rates of 10, 20, and 30 degrees C/min are found to be statistically invariant, the analysis time can be as short as 26 min, which is advantageous for monitoring and control of catalyst performance in an operating unit.