Efficient development of high quality software is tightly coupled to the ability of quickly taking complex decisions based on trustworthy facts. In component-based software engineering, the decisions related to selecting the most suitable component among functionally-equivalent ones are of paramount importance. Despite sharing the same functionality, components differ in terms of their extra-functional properties. Therefore, to make informed selections, it is crucial to evaluate extra-functional properties in a systematic way. To date, many properties and evaluation methods that are not necessarily compatible with each other exist. The property model ontology presented in this paper represents the first step towards providing a systematic way to describe extra-functional properties and their evaluation methods, and thus making them comparable. This is beneficial from two perspectives. First, it aids researchers in identifying comparable property models as a guide for empirical evaluations. Second, practitioners are supported in choosing among alternative evaluation methods for the properties of their interest. The use of the ontology is illustrated by instantiating a subset of property models relevant in the automotive domain.