It is produced with a mixture of cooked glutinous rice, salt-fermented rice malt (koji), and shochu.
There are three types of mirin: 'hon-mirin' (lit. "true" mirin) boasts a complex and mellow flavor with 14% alcohol. 'Shio mirin' or 'Aji mirin' is lower in alcohol as it is not naturally fermented, and is sweetened with glucose and corn syrup as well as 1.5% salt to avoid alcohol tax. The cheapest 'Mirin-fu chomiryo' (lit. "mirin-like condiment') contains less than 1% alcohol and has a salty-sweet flavor that mimics hon-mirin.
It is best to keep opened bottles refrigerated as the cap tends to grow mold in humid environments. It also oxidizes with time and darkens to a deep amber, but the taste should not be affected.