Morphology transitions in the surfactant directed synthesis of mesoporous silica nanoparticles are of great interest as these materials are interesting for applications in catalysis, separation, and drug delivery. The nature of the transition mechanisms often remains unknown, but is vital to understanding of better-designed materials. We investigate a bimodal transition mechanism in the synthesis of single pore silica nanoparticles of two different shapes synthesized through micelle templating. Introducing pore expander trimethylbenzene (TMB) to the system at varying concentrations results in a transition from pure thicker single-pore particles to pure thinner single-pore particles. In the transition region both particles have stable pore and particle sizes while after the transition region an increase in the size of the thinner particles is observed. The bimodal nature of the transition is verified by a combination of gel permeation chromatography (GPC), fluorescence correlation spectroscopy (FCS), dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques.