The OPCW “noted”:http://www.opcw.org/news/article/statement-by-the-opcw-on-syria/ the other day that Syria, as a party to the “1925 Geneva Protocol Protocol for the Prohibition of the Use in War of Asphyxiating,
Poisonous or Other Gases, and of Bacteriological Methods of Warfare”:http://www.un.org/disarmament/WMD/Bio/pdf/Status_Protocol.pdf, “is obligated by international law not to use chemical weapons under any circumstances.”

This I knew. But VERTIC’s Scott Spence and Meghan Brown wrote a “piece”:http://www.vertic.org/pages/posts/syria-international-law-and-the-use-of-chemical-weapons-345.php this past August which discusses some other international legal aspects of CW use, such as the international criminal court and customary international law.

I won’t evaluate their arguments, but take a look. Interesting reading, given current events.

The article also reminds us of this provision of the CWC “Verification Annex:”:http://www.opcw.org/chemical-weapons-convention/verification-annex/part-xi/

bq. In the case of alleged use of chemical weapons involving a State *not Party to this Convention or in territory not controlled by a State Party,* the Organization shall closely cooperate with the Secretary-General of the United Nations. If so requested, *the Organization shall put its resources at the disposal of the Secretary-General* of the United Nations

VERTIC also points out that the UNSG has a Mechanism for Investigation of Alleged Use of Chemical and Biological Weapons. It’s described in “this UN fact sheet.”:http://www.un.org/disarmament/HomePage/factsheet/wmd/SG_Mechanism_Fact_Sheet.pdf