In many cases, highlighter ink is water soluble rather than permanent, like some markers. This makes it easier for soap and water to remove highlighter stains from some surfaces, but exactly how much ink comes off depends on the surface and the highlighter ink's formulation. Some specialty highlighting pens are made to be erasable thanks to either a clear fluid at the opposite end of the writing tip or a special friction-style eraser opposite the writing side. Rubbing alcohol and melamine foam sponges also come in handy for stubborn highlighter stains.
Blot Fresh Stains
If a highlighter leaked all over your favorite shirt, blot the spot as quickly as possible to absorb some of the tinted liquid. Use folded paper towels, holding one set inside the shirt or fabric behind the stain and the other on the outside, pressing from both sides. If the stain happens on a pillowcase or other material where the dye may soak through, separate the stained item from nearby objects to limit the stain.
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For areas such as a couch or even the kitchen table, blot the spot with paper towels, switching to a fresh paper towel as the paper absorbs the highlighter ink. Once you've removed as much as possible, blot again with a damp cloth and then a fresh, dry cloth.
Removing Erasable Highlighter Ink
Take a good look at the offending highlighter to see if it's of the erasable variety. If it is, the end opposite the writing tip should have something similar to an eraser, or it will have another writing tip that produces a clear fluid. The eraser-style end uses friction like a regular eraser, so rub it over the highlighter stain, and it should remove the ink. Test it on an inconspicuous area first to ensure the eraser device won't harm the stained object. It shouldn't, but delicate fabrics may be affected by friction.
If your erasable highlighter uses a clear liquid rather than a friction-style eraser, draw over the highlighter stain using the side of the pen that produces clear fluid. Test it on paper first to ensure it's actually the erasing fluid rather than just a different-size writing tip with the same color highlighter ink. The clear solution reacts with the highlighter ink to remove the original ink color. As with any cleaning solutions, test the clear liquid on an inconspicuous area first if using it on upholstery, the wall, or your clothing.
Scrub With Soap and Water
Highlighter ink comes off fabric and many hard surfaces with dish soap and water. Mix a squirt or so of dish soap with three times as much water, dip a soft, dye-free cloth in the soapy solution, and then gently wipe the stain with it. Work from the outer perimeter of the highlighter stain toward the center to avoid spreading it. If you are working on a thin piece of fabric, such as a shirt, place folded paper towels or an old cloth directly under the stain beneath the fabric to prevent the liquid from soaking through to other parts of the shirt.
Use an old toothbrush to help scrub the stain if the dye won't come out easily. Once you've removed as much highlighter ink as possible, wash a machine-washable item according to the care tag's instructions. For other materials, wipe them down with a damp cloth and then blot dry with a fresh cloth.
Alcohol or Foam Eraser
Rubbing alcohol excels at removing many types of ink from all sorts of surfaces, so it's worth a shot if soap and water don't remove the entire stain. Pour a little on a cotton swab or cotton ball and then blot the stain with the alcohol and watch the ink transfer over to the cotton. Continue with a fresh area of the cotton until you've removed all the ink.
A melamine foam sponge eraser, such as a Mr. Clean Magic Eraser, also gets rid of all kinds of marks and stains, especially on hard surfaces. Wet a small area of the sponge and rub it over the ink and the stain should vanish.