Beam-to-column connections are of great significance as they noticeably influence the mechanical behavior of structures at ambient and elevated temperatures. Observations from full-scale fire tests confirm that connections play an important role on the resistance time of structural components in fire. Because of the high cost of elevated temperature tests, adequate experimental data on a broad range of connections are not available. One type of such connections is the I-beam-to-circular tubular leg connections in offshore oil/gas platform topsides. Considering the high risk of fire events in offshore oil/gas platforms, our study focuses on the structural behavior of this type of connection at elevated temperatures. Eleven small-scale experimental tests were conducted on a uniplanar welded steel I-beam-to-tubular chord connection with external diaphragms to investigate their fire resistance capacity. Local strengthening and partial thermal insulating were separately introduced to the connection components. The results show that the external diaphragms play a considerably more important role on the connection fire response as compared with that for the vertical stiffeners. It is also found that the degradations in the connections' stiffness at elevated temperature might be closely correlated with the classical thermomechanical data on steel material. Copyright © 2013 John Wiley & Sons, Ltd.