Pyridine is since a long time used as an infrared (IR) molecular probe to characterize the acidity of solids, in particular of heteropolyacids (HPAs). The latter are metal-oxygen-clusters nowadays widely used in acid catalysis. Indeed, thanks to their exceptionally strong Bronsted acidity, they allow operating chemical reactions under significantly milder conditions than required by conventional catalysts. In the case of H3PW12O40, the most acidic Keggin-type HPA, it is well-known that the exposure to pyridine leads to an unusual IR spectrum. Indeed, the band at about 1540 cm(-1) associated to the nu(19b) pyridinium ring stretch mode is split in two. Up to now, this phenomenon was studied with the only aim of understanding its origin, which was proposed to be a tunneling effect related to a frustrated rotation of the pyridinium cation within the solid bulk of H3PW12O40. Here, we demonstrate for the first time that it can actually be used as an analytical tool, namely to probe the degree of dispersion of supported H3PW12O40 units, a key parameter dictating their catalytic performance. We support H3PW12O40 on TiO2 P25 (hydrophilic) and TiO2 T805 (hydrophobic), so yielding samples with different degrees of dispersion, as qualitatively assessed through X-ray photoelectron spectroscopy. Through in situ Fourier-transform IR spectroscopy upon pyridine adsorption, we show that the nu(19b) pyridinium vibration band only and systematically splits in the presence of agglomerated Keggin units. More precisely, the higher the fraction of agglomerated Keggin units, the more marked the band splitting. So, the latter appears as a fingerprint of agglomerated H3PW12O40. Furthermore, following an easy treatment, the IR spectra allow quantitatively evaluating the fractions of dispersed vs. agglomerated Keggin units within the samples, which is very difficult to achieve through any other technique.