I'd like to know whether "of" or "on" should be used in the following in a restaurant setting:

Do you offer free refills of / on coffee?

I'd appreciate your help.

closed as off-topic by user070221, Davo, Andrew, user3169, M.A.R. ಠ_ಠ Jan 7 at 9:49

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  • ....free coffee refills? would work just as well. – Ronald Sole Jan 2 at 13:51

Both prepositions are grammatical.

The prepositions can have slightly different meanings, although not every speaker may employ the distinction.

on coffee refers to coffee not as beverage but as (menu) offering, that is, as item for sale.

of coffee refers to coffee as beverage, something that can be dispensed into your cup.

You're familiar no doubt with of (cup of coffee) and don't need examples.

There's a holiday sale on helmets at BikesBikesBikesBikesBikes.com

With on, "Free refills" is cast as retail incentive analogous to "discount".

And thus on can be understood as "applied to" in its figurative, not its sticky literal sense.

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