Is the problem on one stalk - as in, part of the plant is affected, while the rest is healthy? If so, it maybe a fungus. In which case, you'll need to cut off all of the affected parts and repot with fresh soil.
Barb

I found some great sites online that talked about the many diseases that these plants can get.

These are the sites - they give a lot of detail about care for your hibiscus.
http://www.trop-hibiscus.com/gindr.html. http://www.hiddenvalleyhibiscus.com/care/winter.htm

The hibiscus is grown as a standard. The leaves that are turning are on different branches. What's confusing me is that it's getting new leaves on the same branch and they are fine, but some of the older ones are turning yellow. I moved the hibiscus to a closed in porch where it has better lighting, and although it's getting a lot of new leaves, some of the older ones are still getting yellow. The plant as a whole looks healthy.
I checked out the sites that were suggested, but couldn't find any information about the leaves turning yellow.
I've also noticed some gray spots on some of the older leaves.

Gray spots could be powdery mildew, but you didn't say if they were on the leaves when they started to yellow or if they appeared after they yellowed. If before, the mildew would indicate too much moisture on the leaves, if after, the leaves are just decomposing.

Always water from below and try to avoid getting the leaves wet in the evening. I like to water in the morning so the moisture has a chance to dry off during the day. Also, when the plant dries off during the day, the sun kills the spores that cause the mildew. The mold and mildew spores are in the soil and in the air, so you can hardly avoid getting them on the plant's leaves.

The only other thing I can think of, is that leaves don't live forever and maybe your plant just needs fresh foliage. I don't really have much experience with hibiscus, but I do know that sometimes a perfectly healthy plant just drops leaves. Since your plant is putting on new growth, it's probably OK.

I have one hibiscus and it dropped about 10 leaves when I transplanted it after bringing it home from the nursery. But it has been doing fine and put on many new leaves since then. I didn't look at the dead leaves closely to see if they had any sign of disease, but I did remove them from the plant and the area to avoid the possiblity of spreading diseases on them. This is a standard practice I use with my roses, which are very prone to disease, molds, mildew, fungus and pests. And I do have a lot of roses. Cheryl

Some of the leaves that had the gray were not yellow, some of the others had grey spots and yellowing, others were just yellowing. I took all the discolored leaves off.
I also read that it could be a nutrition problem.

Does any one know the name of a name brand fertilizer that has low phosporus? I'm trying to find the right fertilizer for my tropical hibiscus. The ratio numbers are 9-3-13, 10-4-12, and 12-4-18.I've tried a few nursery departments but I haven't found it.

Look for Carl Poole or Diamond brands as two options. Even miracle gro has a 4-1-2 all purpose soluable that can be used and is probably more commonly available. Peter's I believe has a 3-1-2 ratiod fert as well. Overall I am not too concerned with the brand but the ratio and other micros it has in it. The Carl Poole is the best overall in all types that I have found so far and is what I used last year and am still using this year and am happy with the results.