Silica particles are traditionally made via the hydrolysis and condensation of tetraalkoxysilanes with the use of methanol and ammonia as a basic catalyst. More recently, bioinspired polyamines have been used in place of ammonia. Particle formation via the use of tetraalkoxysilanes typically occurs extremely quickly with cloudy precipitates forming immediately, making it practically impossible to characterize the reaction in real time. Our study uses trimethoxymethylsilane (TMOMS) and the polyamine polyethyleneimine (PEI) to form PEI-silica particles via a reaction that takes place over several minutes, allowing us to study the reaction in real time. The acidic hydrolysis of TMOMS and basic polymerization condensation of TMOMS via PEI to form solid PEI-silica particles were observed in situ over time using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and dynamic light scattering (DLS) for the first time. The ATR-FTIR data suggest that dimer formation occurs during acidic hydrolysis followed by PEI-catalyzed condensation to form silsesquioxane structures. The results for the particles formed in situ were then compared with those for particle samples that had been washed to remove excess reactants. The ATR-FTIR results were corroborated via scanning electron microscopy and DLS, which suggest that the growth of PEI-silica particles occurs by aggregation of smaller particles to larger ones, because the data show the presence of small particles and much larger particles at the same time throughout the whole particle growth process.