Hey guys,
I know something like this has already been posted here but I want to share with you guys what I did to revive some old lego pieces that yellowed.

All that I used is 'Vanish Liquid stain remover' and 'Vanish Power O2 stain remover powder' which I bought at Tesco. It can be found at the area where all the laundry detergent is.



Steps:
1. Prepare a container to place the lego pieces in. It's better to use a transparent container with a wide surface area.

2. Pour the Vanish Liquid and one or two spoons of Vanish powder into the container. Can add a bit of water to dilute the mixture in order to fill up the container.

3. Place the lego pieces into the container. Try not to let the pieces touch each other.

4. Place the container under direct sunlight. I whole day of sunlight is enough to revive pieces that is not so badly yellowed. Flip the pieces halfway through the day.

This works best with white and grey pieces. Yellow, blue and red pieces didn't turn out so well as there are minor color fade to a few of the pieces.

The Vanish liquid contains H2O2 and the Vanish powder is an oxi-type cleaner which acts as a catalyst. I think that's why it worked.
The surface of the lego pieces needs be exposed to direct sunlight or UV. That why its not good if a lego piece blocks another piece.
Sorry I didn't take a before-and-after picture to show. I didn't expect to post this.