Cause of RO water aftertaste

[alizarin]

I just recently got a new rodi unit (sediment, catalytic gac carbon, .6u carbon block, filmtec ro, di) and had a question...
I know that the di output is supposed to have an aftertaste but why does the RO-only (di bypass)? It tastes great at first but a minute later there's the aftertaste. If I add a half cup of unfiltered water to a gallon of filtered it tastes fine again.

Can anybody tell me what's causing that?

[BLockamon]

Might be breakthrough on the membrane or some organic by-product of the membrane. Please who use RO for drinking water typically use a final carbon filter to remove the taste.