A number of X-ray sensitive silicon p-i-n diodes have been spectrally calibrated between 1.5 and 17.4 keV using mechanically chopped fluorescent line radiation. The experimental procedures are described and the measurements of diode sensitivity are shown to fit a simple two thickness diode model. As the derived thicknesses generally differ from those quoted by the manufacturer, X-ray calibration is recommended for this type of diode.