stainless steel, this thermal cup will keep your drinks hot or cold for longer than your standard to-go cup. Features a double wall and is vacuum insulated for long lasting temperature maintenance. 2.9in bottom of cup fits in most cup holders, making it easy and convenient to take with you on a trip or on your way to work. Acrylic slide lid is easy to clean and has a rubber ring to secure a tight seal and prevent leakage.