This paper is the second in a series of characterization studies of clay and sediment reference materials (RMs) conducted by the Center for Applied Isotope Studies, University of Georgia, and the Research Reactor Center, University of Missouri–Columbia. Here we report the following major, minor and trace element mass fractions determined using wavelength dispersive and energy dispersive X-ray fluorescence and instrumental neutron activation analysis for three fireclay RMs (c.137–139) manufactured by Mittal Steel Ostrava a.s. and distributed by Brammer Standard Company, Inc.: major elements reported as oxides Na2O, MgO, Al2O3, SiO2, P2O5. K2O, CaO, TiO2, MnO, Fe2O3 and the following minor and trace elements: As, Ba, Ce, Co, Cr, Cs, Cu, Eu, Hf, La, Lu, Nb, Nd, Ni, Pb, Rb, Sb, Sc, Sm, Sn, Sr, Ta, Tb, Th, U, V, Y, Yb, Zn, and Zr.