A setup for optically monitoring the agglomerate growth of multiwalled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition on single Co-Mn-Al-Mg oxide catalyst particles with ethene as carbon precursor has been developed. Ethene concentrations and temperatures were varied between 5 -75 Vol.% and 550-770 degrees C, respectively. It could be shown that the agglomerate growth is rapid and the final diameter is reached after a few ten seconds to about 3 min depending on the reaction conditions. The average enlargement factor of the agglomerates over all experiments was found to be 6.5 +/- 1.2 compared to the original diameter of the catalyst particle. The growth rate is enhanced by both, reaction temperature and ethene concentration. Hence it is concluded that the agglomerate growth rate is associated with the reaction rate of MWCNT synthesis. Short time experiments and analysis of the resulting agglomerates have confirmed an earlier proposed growth mechanism. (C) 2013 Elsevier B.V. All rights reserved.