Thank you, guys.
Well, I decided to make a search and, according to the United Nations Postal Administration website, <<United Nations stamps are issued simultaneously at UN offices in New York , Geneva and Vienna . Each issue carries a related design theme, with different denominations for each office. The stamps are available from UNPA offices in person or by mail, and from stamp dealers. They are valid for postage when used on mail from the UN offices in New York , Geneva and Vienna>>.
Case closed...

As far as I know, UN stamps were issued mostly for the sake of collecting. They are not good for postage within the United States, but are accepted only for mailing within the UN system.

Even if they were, I would say don't use them for your SASE. I think using the forever stamps would be a better idea since you never know how long it will take to get a reply, and while you're waiting the postage rates could increase. If anything, I would use any "regular" postage stamps to send. Every now and then I also come accross some old stamps laying around the house and do use them to send items out, but never for the SASE.

Hmmmm, I don't think those would work, since their United nations and not United States.

RYAN J.

Hi guys,
cleaning home I found a lot of vintage United Nations stamps.
I’m not interested to collect them, so the question is: can I use very old United Nations stamps for SASEs to use in the United States? The stamps I found have variable values, from 1 c to 1 dollar and they all have a “United Nations” inscription, usually in multiple languages.
Many stamps don’t have an english inscription (for example, a 20 c stamp only reads “Naciones Unidas – Nations Unies”…).
I guess it’s a stupid question , but your help would be much appreciated!
Thank you.