Is gasoline volatility a function of octane?

Gasoline formulations are too complicated to make a determination of one particular property based solely on the octane rating. In the old days (really old days), the octane rating of gasoline was adjusted mainly by varying the proportions of different hydrocarbon compounds that can be separated from crude oil by distillation. When this was the case, you could make some judgments about the properties of the gasoline from the octane rating. This all changed with the introduction of tetraethyl lead. This compound boosted the anti-knock properties of gasoline blends without greatly affecting the other properties. Lead is now out of gasoline, but other chemicals have taken the place of lead to some extent. The EPA specifies gasoline characteristics. These are adjusted by refining, blending, and doping the blend with chemicals to modify the characteristics.

Chevron has some interesting information on this subject. See:

Ed White

Richard Schumacher wrote:

CE White is correct that it's pretty complicated these days BUT, as a general guide, higher octane fuels have lower volatility (they evaporate slower). That is one of the reasons Ford issued a TSB on "hard starting" complaints that advised people to stop putting premium in vehicles/engines designed to run on regular gas. The premium gas was sufficiently less volatile that it made the engines hard to start because the cold enrichment (choke) was set for regular fuel, not premium. I have experienced this on my Explorer. When it used to ping in hot weather I would put premium in it. It would then become harder to start. Going back to regular gas returned it to normal easy to start.

I'm not sure if I understand your question but there is no required relationship between volatility and octane. That said, the net result of how they make gas is that the higher octanes do, in practice, have lower volatility. The poor starting is because the fuel enrichment coded into the computer for cold starts is based on the use of regular gas with the expected higher volatility. When you use premium fuel, with it's lower volatility, it has the effect of being too lean for a cold start, not because it's not pumping out the fuel, but because it doesn't vaporize/atomize as quickly and become available to burn. So Premium fuel = less available atomized gas when cold = harder to start = effectively too lean