Abstract The characterization of a high performance, energy dispersive Si(Li) solid-state detector is presented. The response of the detector to K and L lines in the energy range 0.7–9.0 keV is obtained by using a Van de Graaff accelerator to bombard selected targets having 12 ⩽ Z ⩽ 50 with a high energy proton beam. In addition to the channel-to-energy calibration and resolution vs energy plot, an empirical relation is presented for line yield as a function of energy for the K lines. Further, spectral data are used to estimate the silicon dead layer thickness. Finally, a detailed characterization of the detector's response function, including deviations from a simple Gaussian form due to effects such as incomplete charge collection, is given. From these results, a procedure is indicated by which X-ray spectrometers used in X-ray astrophysics applications may be calibrated.