Before I get to why there is condensation inside your bottle of spirit — or decanter — let’s clear one thing up: If you do see condensation, it’s not a bad thing, or a sign that somehow the spirit inside has gone bad. So if you’re concerned, don’t be.
Here’s what’s happening: Condensation forms when the air inside the bottle is cooler than the air outside of it. This results in a lower saturation point for the air and liquid, which displays as condensation on the glass inside the bottle. This is very common, especially if you have your bottles sitting on a counter or shelf, as the temperature surrounding the bottle will fluctuate as your air conditioner or heating unit turns on and off. Condensation is also a pretty good sign that the seal on that bottle is tight. The tighter the seal, the harder it is for the air inside it to escape, thereby creating the condensation.