In the present work, electrospray ionization mass spectrometry (ESI-MS) was used to investigate the titanium clusters present during nucleation and growth of sol-gel derived TiO2 nanoparticles. Depending on the alkoxide precursor used, Ti clusters of different sizes were identified [titanium isopropoxide (TTIP)-11-12 Ti atoms, titanium butoxide (TTB)-10-11 Ti atoms and titanium tetraethoxide (TTE)-5-7 Ti atoms]. The Ti-O-Ti backbone/core of the titanium clusters were found to be quite stable after formation and do not easily break up into smaller clusters. The results of this investigation suggest that these clusters are used as building blocks in the growth during the induction period. The h-ratio (n H2O/n alcoxide) and the pH were not found to influence the identity of the Ti clusters present during nucleation of the growth of sol-gel derived TiO2 nano-particles, although the induction period was greatly influenced. The reactivity of the alkoxy group are OEt > OPri > OBu. The difference in reactivity of the alkoxides is generally believed to be due to the increasing size of the alkoxy group which causes sterical hindrance during the nucleophilic substitution reaction.