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Residue conversion processes are becoming increasingly important in the world today, because of several market and economic factors. The residues contain a large mass fraction of heavy and polar compounds known as asphaltenes, which can cause major problems in the conversion processes. Heavy feedstocks with high asphaltene content can become an economic opportunity if they can be converted to lighter products. In this perspective, Eni has developed a process called EST (Eni Slurry Technology) to convert very heavy feedstocks into high quality transportation fuels. In the course of the EST process, the asphaltene molecules are converted to smaller fractions, enhanced to fuels; the unconverted fraction is recycled in the reactor and treated again and again. The technology is based on a very active, dispersed, nonaging slurry catalyst, which prevents coke formation and promotes upgrading reactions (sulfur, nitrogen, and metal removal and Conradson carbon residue (CCR) reduction). Knowledge of the asphaltene molecular structures and their evolution during the process is important to improving the treatment capability; unfortunately, being a solubility class, the presence of a huge multitude of different species (heavy and highly aromatic in particular) eludes their chemical identification. One of the most difficult parameters to obtain is the (average) molecular weight (MW) of asphaltenes, which has been controversial for decades: the main problem is their "strong" tendency to form aggregates due to the π−π aromatic interactions between large polyaromatic sheets; also, the presence of heteroatoms can have a great influence on the aggregation of the asphaltenes. Two techniques commonly used to determine MW are vapor pressure osmometry (VPO) and gel permeation chromatography (GPC). Both suffer from limitations due to the aggregation of asphaltenes; GPC also shows the tendency to slow down the elution of pericondensed polyaromatic hydrocarbons, yielding defective MW values. Recently, two new techniques have been proposed: atmospheric pressure photoionization-MS (APPI-MS) and time-resolved fluorescence depolarization (TRFD). We have applied GPC, APCI-MS, and TRFD to three different asphaltene samples, one obtained from the vacuum residue of a native crude and two from the corresponding products after EST processing under different severity conditions. The comparison of the MW values obtained provides some insight into the advantages and limitations of the techniques. All techniques show the reduction of asphaltene MW after hydrotreating, in accordance with the expected reduction of alkyl chains. The same asphaltene samples studied by total scanning fluorescence (TSF) showed a transformation of their aromatic cores during the EST process.