This metaphysical treatment of matter, as categorised by chemical concepts bearing on distinctions between homogeneous and heterogeneous matter, and between one or more substances, discusses the modal and mereological features of phase and substance predicates needed to describe mixtures from a macroscopic perspective. These predicates are relational, applying to a quantity of matter at a time. So is the “same substance” predicate, which doesn’t impose restrictions of phase (e.g. water’s being liquid, allowing that “All H2O is water” is true as well the converse). Understanding quantities to satisfy the conditions of extensional mereology, Quine’s view of the distributive condition on mass predicates is disputed. This leads to an analytic comparison between the Aristotelian and the Stoic conception of mixtures, which are developed as alternative responses to the distributive condition calling on the rigidity of quantities. Analogues of the rigidity conditions are discussed for several key predicates in the discussion.—Correspondence to: paul.needham@philosophy.su.se