If you are trying a fragrance, 100 ml. If you have found the one, 500 ml, it is five bottles of oil for $135 instead of $240, and it is the only size you can subscribe to. The Twin Pack sits between: two 100 ml bottles for $90, same scent or two different ones.

Yes, in any cold-air diffuser. The oil goes into the reservoir, not into a proprietary cartridge, so brand does not matter.

No. Water and ultrasonic diffusers need Water Diffuser Drops, which are made to disperse in water. This is pure oil and will sit on the surface.

No. That system takes the A/C Scent Refill, a pressurised can rather than a bottle.

Yes. Swap the oil, clean the bottle, and the new scent starts on the next cycle.

 It depends entirely on your diffuser's output and how many hours a day you run it. Check your device's run time per ml , the same bottle can last a month in one machine and a week in another.

You can, though each one is composed to be complete on its own. If you want to move between two, the Twin Pack lets you take one of each.

To the 500 ml, yes, it arrives on your schedule with 20% OFF & Free Shipping, and you can skip, change frequency or cancel any time.

It is cosmetic grade, vegan, cruelty free and formaldehyde free, diffused as a fine atomised mist rather than a heated vapour. As with any fragrance keep the bottle out of reach.