In article m, "John" wrote:
I built an adjustable fixture that allows me to make very accurate loose
mortise and tenon joints. I have been using tenons fabricated from hardwood,
either Oak or Poplar.

Very different physical characteristics there. Much better IMHO to make the
tenons from the same wood they will be used to join, so that the tenon expands
and contracts at the same rate as the mortises.

Needless to say, it requires some work to achieve the
1/4 inch thickness required for a tight joint.

"Needless to say"?? Apparently you don't have a thickness planer available to
you; with a good thickness planer and a modest amount of practice, achieving
any particular thickness is quite easy. You don't even need precision tools to
measure very small variations from the desired thickness. If the desired
thickness is 1/4", plane four boards to the same thickness, then stack them
and measure the combined thickness. If the stack measures 1-1/32", you know
each board is 1/128" oversize. If a stack of eight is anywhere between
1-31/32" and 2-1/32" then you know each board is 1/4" +/- about 0.004".

I am now considering using 1/4 inch tempered hardboard. It is stable, strong
and has a uniform thickness. Any have experience with this technique??

I wouldn't even dream of using hardboard, tempered or otherwise, for floating
tenons. The whole point of a mortise-and-tenon joint is its strength, which
would be significantly compromised by the substitution of hardboard for solid
wood. Additionally, the very fact that it *is* stable is another reason it's
unsuitable for this purpose. The wood that you're joining will expand and
contract with changing humidity, while the hardboard will not, thus
compromising the glue joint. Best practice is to make the loose tenons from
the same wood that they are used to join.