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The mechanism of fluorescence enhancement of Tb3+ in Tb3+‐trimesic acid‐Gd3+ systems has been explored using dynamic light scattering. In earlier work, the mechanism of fluorescence enhancement of Tb3+ in these systems was proposed to be due to the formation of a polynuclear complex involving Tb3+, trimesic acid and Gd3+. However, no direct experimental evidence was presented for the mechanism in that work. In this work, we have used dynamic light scattering (DLS), to measure changes in the mean hydrodynamic diameter (Zavg) of the Tb3+‐trimesic acid‐Gd3+ complex, as a function of time. The Zavg was found to increase monotonically as a function of time, indicating a continuous growth in the particle size. The Eu3+‐β‐diketones‐Gd3+ system also shows enhancement of Eu3+ fluorescence; though a different mechanism was believed to be operating in this case; namely intermolecular energy transfer. If the two systems followed different mechanisms for fluorescence enhancement, the Eu3+‐β‐diketones‐Gd3+ system, is expected to show a different time evolution of particle size. The DLS study showed that the Zavg increased rapidly to a maximum value and then remained nearly constant throughout our measurements; that is, no gradual increase in Zavg was seen as in the Tb3+‐trimesic acid‐Gd3+ system. These DLS studies of the Tb3+‐trimesic acid‐Gd3+ and Eu3+‐β‐diketones‐Gd3+ systems, therefore confirms that different mechanisms of fluorescence enhancement are operative in the two systems. The study has demonstrated that the increase in fluorescence intensity with time when Gd3+ is added to the Tb3+‐trimesic acid complex is accompanied with an increase in particle size as revealed by dynamic light scattering studies. These observations are explained invoking the role of polynuclear complexes in the mechanism of fluorescence enhancement. [ABSTRACT FROM AUTHOR]