This paper presents an experimental and numerical investigation into the post-fire behaviour and residual capacity of S700 high strength steel circular hollow section (CHS) beam-columns. The experimental investigation was performed on ten S700 high strength steel CHS beam columns and included heating and cooling of the specimens as well as post-fire material testing, initial global geometric imperfection measurements and pin-ended eccentric compression tests. A subsequent numerical investigation was conducted, where finite element models were developed and validated against the test results and then employed to carry out parametric studies to generate further numerical data over a wide range of cross-section dimensions, member lengths and loading combinations. In view of the fact that there are no specific provisionsfor the design of steel structures after exposure to fire, the relevant room temperature design interaction curves were evaluated, using post-fire material properties, to assess their applicability to S700 high strength steel CHS beam-columns after exposure to fire, based on the test and numerical data. The evaluation results revealed that the interaction curves provided in the American Specification and Australian Standard result in a high level of design accuracy and consistency, while the Eurocode interaction curve leads to more conservative and scattered failure load predictions. Finally, a revised Eurocode interaction curve, with more accurate end points, was proposed and shown to offer improved failure load predictions for S700 high strength steel CHS beam-columns after exposure to fire.