CAS Numbers The Chemical Abstracts Service (CAS) has developed a registry of unique numeric identifiers, The  CAS Numbers or CAS Registry Numbers  , for more than 73 million organic and inorganic substances including compounds, metals, alloys, minerals, organometalics, elements, isotopes and biochemicals. Each number represents only one substance and has no chemical significance but serves solely as an internationally observed substance identifier by scientists, industry and regul...