The in situ incorporation and characterization of 2-(2'-hydroxyphenyl)benzothiazole (HBT) in the cages of nanosized FAU zeolites is reported. We demonstrate the advantage of using colloidal zeolite solutions to perform subpicosecond transient experiments on nanosized host/guest systems. FAU molecular sieve is prepared from precursor solutions containing as organic template only tetramethylammonium hydroxide (TMA) or both molecules HBT and TMA, using a hydrothermal treatment at 90 degreesC for 70 h. In situ dynamic light scattering investigations of the precursor solutions and the crystalline suspensions are performed with the original sample concentrations using a backscattering mode. The radius of the amorphous entities formed in the TMA-containing precursor solutions is about 25 nm, while that of the amorphous species in the HBT/ TMA precursor solution is about 15 nm. The final particle size of FAU and HBT/FAU colloidal zeolites is 100 and 80 nm, respectively. The encapsulation of HBT with different concentrations into the large pore FAU molecular sieve host is confirmed by Raman, infrared, and C-13 solid-state NMR spectroscopies. The spectroscopic data reveal that the HBT molecules are incorporated in the nanosized zeolite, particles, thus leading to changes in the environment of the TMA ions as well as in the local atomic arrangements of the FAU structure. At high concentration of HBT, a large fraction of the sodalite cages are destroyed, and the HBT and TMA molecules are located in the subsequently formed cavities. Steady-state UV-vis spectra also reveal the presence of the keto HBT-conformers inside the FAU zeolite nanocrystals. Upon UV excitation, the HBT molecules occluded in the zeolite nanoparticles undergo ultrafast intermolecular proton transfer within 1.5 ps.