Gold nanorods are mostly synthesized by chemical methods, using a directing agent: CTAB (cetyltrimethyl-ammonium bromide). Due to the various applications of gold nanorods, the necessity to modify the surface functionalization appears. Thiol containing molecules are used as direct or cross-linking agents in the functionalization of gold nanorods. In this study we show that a thiol containing carboxylic acid mixture is able to remove a large part of CTAB molecules that surround the gold nanorods, and not only to cover the CTAB protecting layer. Using a simple FTIR spectroscopic method, it is possible to quantitatively determine the percentage of CTAB that is replaced by the functionalizing agent. (C) 2013 Elsevier B. V. All rights reserved.