Different mathematical models based on electrochemical permeation tests have been proposed to determine the density of hydrogen trapping sites (N-T) in metals. However, recent mathematical models that have been used to determine N-T contain mathematical errors or have been based on the work of researchers whose works contain no mention of equations or mathematical terms related to hydrogen trapping, which calls into doubt the validity of the models' applicability. In this work, based on permeation test results for X65 steel, we show that the use of incorrect mathematical models leads to values of N-T that differ by four orders of magnitude. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.