The noise of a single-stream circular jet and that of a coaxial jet with coplanar nozzles of 2·5 area ratio have been measured under simulated flight conditions in the RAE 24 ft wind-tunnel. The majority of tests were conducted with the single-stream jet and primary section of the coaxial jet at a nominal temperature of 880 K. The data have been used to quantify the effect of jet temperature and were combined with measurements from an earlier test series to establish a prediction method for the effect of flight on the noise of single-stream subsonic jets. This method is based on jet noise theory modified by experimentally derived constants. For coaxial jets it is concluded that the noise reductions, which are independent of the secondary stream velocity, are predicted to an acceptable degree by the method suggested for unheated single-stream jets. The prediction methods are suitable for both OASPL's and spectra.