Ok, humble pie time..
I was wrong - plant them trees! Well maybe just the one anyway.
From the Bosch Automotive Handbook:
"When combustion of pure fuel is complete and ideal, i.e. complete combustion of fuel and oxygen without any unwanted secondary reactions, only the following would be produced:
-Water
-Carbon dioxide"
It's been a few years since I worked on emissions (back when no one really cared!).
Ooops!
JC