Abstract: This invention consists in a flexible, very light and pocket-size container, which can keep the temperature of a liquid inside a bottle (4) even. The bag (1) is made by folding a rectangular foil in two and then welding two sides, leaving only one side open through which the bottle (4) or the food is inserted. The foil is made up of thermal-insulation, reflective and waterproof materials. Two holes (2.1, 3.1) having the same dimensions as a bottle neck (4) are located near the open shorter side. An elastic band (8) closes the isothermal bag (1) after rolling it up or folding it up thus making it compact and pocket-size. The bag (1) wraps up the bottle (4) completely but the holes (2.1, 3.1) enable users to drink without opening and closing the bag (1). Its closing device is functional, hygienic and user-friendly.