You're right, it is a mixture. Here's Wikipedia's definition:
"Syngas (from synthesis gas) is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen generated by the gasification of a carbon containing fuel to a gaseous product with a heating value."
http://en.wikipedia.org/wiki/Syngas
I found the last sentence interesting:
"However, the total energy efficiency is not very high, if the gas is used to make fuel, meaning that the purification processes are very energy intensive."