The sulfur specification for diesel fuel has been tightened exponentially over the years. In the near future, the specification will go down below 10 ppmw or less. A fundamental question then is whether the goal of reaching such ultralow sulfur regime will scale exponentially in difficulty. To help answer this question, this paper aims to identify limiting factors bearing on the chemistry and catalysis in this regime. The emphasis is on low-pressure operation. Among the topics discussed are: reactivity-property relationships, catalyst functionalities, inhibiting effects, poisoning dynamics, nature of active sites, and bulk transition metal sulfides. It is shown that certain bulk metal sulfides are intrinsically active and selective for desulfurizing 4-substituted and 4,6-disubstituted dibenzothiophenes. (C) 2004 Elsevier B.V. All rights reserved.