The design of formulated products, such as cosmetic emulsions, requires handling heterogeneous information such as ingredients data, property models, heuristics, and theoretical knowledge, among others. Because of this, the formulation process is not straightforward, but iterative, highly intuitive, and very experimental. Our proposal to facilitate the design process is an ontology called OntoCosmetic for emulsion-based cosmetic products. An ontology is a conceptual knowledge representation of a domain. It can be interpreted by both designers and machines, allowing extensive exploration of the solution space and a logical understanding of the results of that exploration. This article presents in detail the construction of OntoCosmetic, whose structure is based on a domain ontology for Process System Engineering called OntoCAPE. Two examples show its use: the substitution of an ingredient in a formulation, and the exploration and delimitation of the solution space in a new formulation design problem.