What could this be?

When I look through my viewfinder, I see a noticable dark spot positioned in the lower-right quardant (see drawing). I cleaned my eyepiece but it is still there. Nothing shows on any of the pictures I take. Could this be a mirror spot? Anyone ever seen this before??

It could be anywhere on the optical path from your mirror to the inside of your viewfinder glass. It won't affect your image.

As to trying to clean it, well, one fast thing you can try is to pick up a bulb "blower" such as a Rocket blower, and then without physical contact blow off the mirror and see if that helps, and you can direct some air at the focus screen beside the mirror.

There are some special brushes that can be used with the mirror and even the focus screen, but that requires a very light touch.

From there, well, you may or may not have a camera with a focus screen that can be removed by you and cleaned, or you may not.

And then, there is the rest of the optical path which is internal, and so, well...you could try a Canon service center, but I don't know how good they are at those tasks.

Thanks Tony. Kind of what I thought. I have a 5D2, so I'm not inclined to do much OJT on working on the innards. I may try the gentle blower, since I do have one.

Similiar thing happened to my T3I and it was a focus screen/viewfinder issue. Tony was on the money with his points. I attempted to get it out by removing the screen and it was to no avail. Got annoyed since the camera was new so I sent it back and Canon repaired it.

If it is sharp and in focus I would speculate that it is on the focus screen. A spot on the mirror or on the penta-prism wouldn't be in focus I would think.