The question is what your taxable basis is in the miles. For most miles, this is zero. For miles won in a contest where you paid taxes, you could very plausibly argue that your basis was the amount of income you paid tax on according to the 1099 or whatever other valuation method you used. In the case of lancemib, if you're accepting the valuation of 2.5c/mile that Vonage is using for income tax purpose, then you could definitely argue that you could take a 2.5c/mile deduction for donating them since you have a 2.5c/mile taxable basis in them. But this case is very much the exception rather than the rule - certainly miles earned from flying have a zero tax basis in the US. And lancemib certainly would have to argue a bit with the IRS if they audited since the case is so unusual.