<p>The surface tension of polymers under ambient or high-pressure conditions has been extensively studied but is less explored under high vacuum. Here, the effect of air pressure on the surface tension of polymer melts is studied by using a home-built apparatus. While previous studies showed that the surface tension of most polymers decreases linearly when the temperature or pressure is increased, we show that the surface tension of polymer exhibits an anomalous behavior: The surface tension remains a decreasing function of the temperature, but increases with increasing air pressure. The measured surface tension of several polymer samples is well described by the Hill equation in the pressure range of <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({10}^{-4}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mo>−</mo> <mn>4</mn> </mrow> </msup> </math></EquationSource> </InlineEquation> to <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({10}^{5}\,{{{\rm{N}}}}/{{{{\rm{m}}}}}^{2}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>5</mn> </mrow> </msup> <mspace width="0.25em" /> <mi mathvariant="normal">N</mi> <mo>/</mo> <msup> <mrow> <mi mathvariant="normal">m</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msup> </math></EquationSource> </InlineEquation>. The implication of this behavior on the air-polymer interaction is discussed within the context of air adsorption</p>