Vitamins are important scientific categories in different contexts. This paper argues that vitamins are investigative kinds in middle-range ontologies: categories subject to open-ended investigation and that track features of the world. Section 2 presents the history of vitamin discovery to illustrate how the introduction of the “vitamin” category and subsequent research led to the identification of many different vitamins. Section 3 explores whether vitamins can be considered natural or conventional kinds. Section 4 argues that vitamins are investigative kinds. Section 5 considers the ontology of vitamins as investigative kinds in a middle-range ontology.