How do I clean a white substance on dark grout?

You can try vinegar and a small brush (I use an old toothbrush) to clean it. If it's tile in the shower area, it's probably soap scum and vinegar works wonders on it.

It could be hard water deposits, you could try CLR if the vinegar doesnt work.

Baking soda and small brush Make a paste leave on then wash off

A solution of half water and half vinegar is also effective at cleaning both dark grout and white floor grout. Fill a spray bottle with the solution and apply directly to the grout. Give it a few seconds to penetrate the dirt and wipe clean with a damp cloth. Repeat as necessary.

Hi Cathy, I would try straight vinegar and a toothbrush, i's likely a calcium build up.

Hello Cathy try a solution of half water and half vinegar is also effective at cleaning both dark grout and white floor grout. Fill a spray bottle with the solution and apply directly to the grout. Give it a few seconds to penetrate the dirt and wipe clean with a damp cloth. Repeat as necessary.

I am going to try the vinegar/water solution and see what happens. Thank all of you for responding!