The in-orbit PHT-S wavelength calibration was performed by measuring emission line objects with PHT-S: see Section 4.6.2. The accuracy in the central wavelength for each pixel is somewhat better than one tenth of a pixel except for a small part of SL where the uncertainty is marginally larger. The accuracy of the wavelength calibration is discussed in the `ISOPHOT Calibration Accuracies' by Klaas et al. 2002, [26] document.