How do you remove the black glue left from old vinyl tiles?

I have two different methods I use for this, and one or the other usually always works. First, I would try spraying W-D40 on it (heavily), then spray a paper towel with some more (on one side) and let it sit for a while. I have never had a glue, new OR old, that didn't dissolve with it. You might have to do it more than once, and you can usually scrape off the stuff that's already dissolved with a putty knife. I do a lot of home improvements (this is my 3rd house) and I usually have great success with this. Another way that I've had work is to saturate the area of glue with pure vinegar, and cover with a vinegar saturated paper towel (as above.) Lots of the old glues were made a lot different than they are today, and I've had success with this also, depending on the type of old vinyl glue they used. (Believe it or not, a sponge saturated in vinegar and left on, removed 50 year old paint flecks from a hardwood closet floor I was redoing...lifted them right off and didn't harm the floor!) Vinegar works well for lots of removal problems. Good luck Mr Handyman!

get GOO-GONE, it works on all gummy stuff. Its great and you can bye it at most stores. I got mine at WalMart.

According to a friend of mine in construction, the only way to remove it is with a rotary sander. Sounds to me like that would be easier than what you are doing and you can rent one so you wouldn't even have to buy it. Good luck!

would you mind telling if it's concrete or wood subfloor.

Just remove it.