Despite the importance molecular structure has acquired in 20th century chemistry, more traditional macroscopic notions in terms of a continuous concept of matter continue to play a role in chemical theorizing. In the light of the extensive and determined criticism of reductionism in recent philosophy of chemistry, it is of interest to see macroscopic ontology treated autonomously. One aspect of this is developed here, namely, the concept of chemical substance. This is characterized by contrast with phases and solutions. The key conception is that of an intensive property, which is defined by appeal to the mereological structure of parts of the entities bearing substance properties.