A substantially permanently buoyant balloon suitable for use as a toy or in advertising, for example, is provided which includes an envelope containing a lighter than air gas such as helium, which envelope is made from attached panels of a nonelastomeric polymer sheet material carrying a continuous metal layer on at least one side thereof. The metal layer can either be a thin metal film contiguously bonded to the polymer, or can be metal which is vapor deposited over the surface of the polymer. The balloon itself is constructed so that the ratio of its volume taken to the two-thirds power divided by its surface area is in the range of from about 0.21 to about 0.01, and the weight of the envelope can range from about 2.6 .times. 10.sup.-4 gm/cm.sup.2 to about 1.7 .times. 10.sup.-2 gm/cm.sup.2. The envelope is preferably manufactured in two dimensional "lay-flat" form in any desired two dimensional shape unlike conventional balloon envelopes which are inherently three dimensional surfaces. Balloon envelopes having the above described volume to surface area ratio and made with the above described composite panel material will be maintained buoyant for an indefinite period of time when filled with the lighter than air gas.