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Abstract: Systems of chemical reactions are often studied at the level of stoichiometry, i.e., as formal transformation of the form 2A+B -> C+D. This begs the question whether all such reactions systems can be understood as models of chemistry. As it turns out, conservation of mass and energy imply non-trivial constraints, which can be characterized completely. The large remaining class of systems is then shown to admit representations in terms of both sum formulas and structural formulas. This observation suggests a number of new questions regarding compositionality and decomposability of chemistry-like reactions systems. As one example I will briefly consider the questions of collective autocatalysis.