Incomplete charge collection in semiconductor X-ray detectors due to carrier trapping is recognized as an important source of signal broadening. In this paper we show the results of calculations of energy resolution for a TlBr detector using an analytic approach developed in our earlier work in which fluctuations in the distribution of photon absorption sites are related to fluctuations in the collected charge. Using measured values of transport parameters for electrons and holes in the detector material we obtained excellent agreement with experiment in the X-ray energy range 6–660 keV.