We report on the in-orbit calibration of staring and chopped observations performed with the spectrophotometer ISOPHOT-S. Staring observations are calibrated using a Spectral Response Function which depends both on the signal and the elapsed observing time. Chopped observations are calibrated with a signal dependent Spectral Response Function, which was obtained from a set of observations of standard stars covering a range in flux densities. Both methods are consolidated, implemented in the Offline Processing software, and produce photometric results with a general accuracy of 10% or better.