A history of using and applying Ho's pseudo-second-order kinetic expression for adsorption systems is presented. A reference section in a paper is important to researchers interested in the paper's statement and in following the study or finding useful information from the paper. This section is as important as the core of a paper. This comment offers information on citing the original idea of pseudo-second-order kinetic expression. (C) 2004 Elsevier Inc. All rights reserved.