This note points out some errors in the proof of the main result of Ding and Chen [2005. Identification of Hammerstein nonlinear ARMAX systems. Automatica 41(9), 1479-1489] and mentions the well-known results that have solved the problem discussed in Theorem 1 of the above paper by Ding and Chen. (c) 2007 Elsevier Ltd. All rights reserved.