As part of the extensive ground testing of the AXAF, x-ray detectors of standard design were used as secondary absolute calibrators. In order to realize the overall facility calibration goal, these detectors need to be calibrated in turn on a primary x-ray standard. This exercise consists of accurately determining the detectors' response to monochromatic x-rays of different energies. Our data were obtained on various beamlines at the BESSY synchrotron storage ring, which is a very accurately calibrated standard x-ray source. Here we report progress in understanding the response function of the solid-state devices (SSDs). The response is modeled using JMKmod in the XSPEC package, and includes effects of pileup, incomplete charge collection, fluorescent escape, and other secondary processes. Knowledge of the SSD response function will permit precise removal of SSD detector effects form AXAF calibration data, and therefore allow accurate calibration of the AXAF system's effective area and other performance parameters.