This Technical Report addresses the specifications of natural gas as a compressed fuel for vehicles as an addendum to ISO 15403-1. Specifically, it is intended to satisfy requests for quantitative data. The following are critical items regarding gas composition: ? water content; ? sulfur compounds; ? particulate matter; ? higher hydrocarbons; ? CO2; ? free oxygen; ? glycol/methanol; ? oil content ? corrosive components; This Technical Report pertains only to compressed natural gas as it enters the fuel containers on the vehicle. It does not apply to the natural gas delivered to a refuelling station. Any alteration of the natural gas supply composition to meet these specifications is clearly the responsibility of the refuelling station operator.