The synthesis of cyclopentylsilsesquioxane a7b3 [(c-C5H9)(7)Si7O9(OH)(3)] was followed by means of electrospray mass spectrometry (ESI MS) and in situ attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy in combination with chemometric analysis methods. This study allowed us to identify silsesquioxane species present in the reaction mixture and to propose a mechanism for the formation of silsesquioxane a7b3. The spectroscopic results proved the existence of short-lived, low molecular weight silsesquioxane precursors and indicated that the final product silsesquioxane a7b3 was only present in very low concentrations in the reaction mixture, suggesting that the high selectivity of the reaction toward silsesquioxane a7b3 is caused by its low solubility.