Table in Appendix gives sensitivity values for ISIS and FOS-2 measured from wide slit observations of spectrophotometric standards, in the case of ISIS with the low dispersion (158 line/mm) gratings. These measurements give the overall performance for the entire system, including atmosphere, telescope, spectrograph and detectors. The performance is expressed as a magnitude (AB magnitude in Oke's system; see for instance Oke, Astrophys. J. suppl. 27, 21, 1974) which gives a measured count rate of 1 photon/second/Å at the detector.