Peter,
Ethylene dibromide is present in avgas to scavenge the lead and prevent it fouling your plugs. Ethlene glycol is a product of photoinitiated decomposition and is not as far as I know a constituent of healthy avgas.
In the combustion chamber ethylene dibromide will decompose during the burn to hydrogen, carbon and bromide ions, it being too hot for ethylene glycol to exist, let alone form. The hydrogen and carbon then just burn with the rest.
Water is not miscible with healthy avgas to an extent that is problem. It separates out and can be drained off safely. Water will only mix if avgas is contaminated with surfactants. If water is left in avgas for a long time microbial growth is possible, but not likely due to fuel toxicity, at the water/fuel interface.
Summary of what I have learned about avgas form this thread.
Keep it clean, keep it dark and cool, keep the storage tank clean and preferably insulated. Drain off water and don't keep it for more than a year. Check it regularly. Filter it properly on the way to your aircraft. If its not clear and blue, or you are not sure about it, dont burn it in your aircraft.
Last edited by Gaseous; 26th Apr 2005 at 22:42.