When rubber balloons are filled with helium so that they float, they typically retain their buoyancy for only a day or so. The enclosed helium atoms escape through small pores in the latex which are larger than the helium atoms. Balloons filled with air usually hold their size and shape much longer.

Even a perfect rubber balloon eventually loses gas to the outside. The process by which a substance or solute migrates from a region of high concentration, through a barrier or membrane, to a region of lower concentration is called diffusion. The inside of balloons can be treated with a special gel (for instance, the polymer solution sold under the "Hi Float" brand) which coats the inside of the balloon to reduce the helium leakage, thus increasing float time to a week or longer.

Beginning in the late 1970s, some more expensive (and longer-lasting) foil balloons have been made of thin, unstretchable, less permeable metalized plastic films. These balloons have attractive shiny reflective surfaces and are often printed with color pictures and patterns for gifts and parties. The most important attribute of metalized nylon for balloons is its light weight, increasing buoyancy and its ability to keep the helium gas from escaping for several weeks.