The second‐order contamination in the BNL neutron crystal spectrometer is measured for the NaCl(220), NaCl(240), Be(101¯1), and Be(123¯1) reflections over the energy range 0.08 to 5 ev. Reflectivity calculations by Holm for neutrons incident on thin crystals of Be and NaCl are verified experimentally. A method is suggested for rapidly calculating the second‐order fraction of the beam using a simple count rate spectrum and the calculated crystal reflectivities.