The purpose of this study was to develop an accurate method for the determination of bromine in polymer materials using X-ray fluorescence spectrometry when the thickness of the sample is less than the bromine critical thickness (t c ) value. This is particularly important for analyzing compliance with the Restriction of Hazardous Substances Directive. Mathematically and experimentally estimated t c values in polyethylene and cellulose matrixes were up to several millimeters. Four methods were developed to obtain an accurate result. These methods include the addition of an element with a high mass absorption coefficient, the measurement of the total bromine contained in a defined volume of the sample, the exploitation of tube-Rayleigh line intensities and using the Br-Lβ line.