The ultrafast deprotonation of 2-(2′-hydroxyphenyl)benzothiazole (HBT) hosted in nanometer-sized FAU and MFI zeolites is reported. Samples are prepared via in-situ incorporation of HBT in the precursor colloidal solutions resulting in the formation of nanometer-sized zeolites under hydrothermal treatment. The diameter of the zeolite particles formed in the crystalline suspensions is determined by dynamic light scattering and high-resolution transmission microscopy to lie in the range 40–100 nm. It is shown that the HBT loading does not influence the degree of the zeolite crystallinity but does change the size and the morphology of the individual zeolite nanoparticles. Colloidal suspensions containing the crystalline nanoparticles are well suited for optical investigations since they are sufficiently transparent and clear. The photochemical properties of the HBT guest in the zeolite-host systems are studied with femtosecond transient transmission spectroscopy. Depending on the acid–base properties either the enol or the keto tautomer of HBT is found to be hosted in the internal voids of the zeolites; upon UV excitation, the HBT-keto tautomer is converted to the enol form in both MFI- and FAU-type hosts. The HBT photoconversion takes place via an ultrafast deprotonation within 1.5 ps as detected by femtosecond transient absorption spectroscopy.