Countering the proliferation of weapons of mass destruction to non-state groups has been a crucial goal for the international community in preserving global peace and stability. Nevertheless, recent allegations suggest that radical jihadi groups have gained access and used unconventional weapons in the Middle East. Namely, there are statements and reports by intelligence officers, politicians and experts that the Islamic State of Iraq and the Levant has gained access to internationally prohibited weapons. By systematically using open source intelligence (OSINT) this paper will assess Islamic State's weapons of mass destruction capability. It will look at allegations in relation to the possession and use of weapons of mass destruction. It will argue that open source information reveals that the group has produced and employed crude forms of sulphur mustard. Moreover, it will show, based on two case studies that OSINT provides valuable information in examining incidents where chemical weapons have likely been used. Before presenting its arguments and findings the paper briefly defines open source intelligence and analyses areas where it provides value. It also examines ISIL's intentions to develop and utilize weapons of mass destruction.