The refining industry uses hydroprocessing to remove metallic poisons from oil during upgrading. Every hydroprocessing catalyst has a specific capacity for individual metal poisons and a collective capacity for multiple metallic poisons. Catalyst metal poison capacity knowledge is developed from spent commercial hydroprocessing catalyst samples. This paper discusses the development of catalyst silicon and arsenic capacity knowledge from delayed coker naphtha hydroprocessing spent catalyst samples and the application of this knowledge in catalyst system design. Methodologies for determining catalyst metallic poison capacities and reactor metal deposition profiles from the chemical analysis of spent catalyst samples are discussed.