Microspheres serve as convenient substrates for studying DNA activity on surfaces. Here, in addition to employing conventional sample preparation involving multiple wash and resuspension steps prior to flow cytometry measurements, we also directly sampled the reaction volume to acquire in situ measurements of primary and competitive hybridization events. Even in the absence of post hybridization wash steps, nonspecific binding events were negligible and thus allowed for direct, quantitative assessment of hybridization events as they occurred on colloidal surfaces. The in situ results indicate that primary duplex formation between immobilized probes and soluble targets on microsphere surfaces is less favorable than predicted by solution models. The kinetics of competitive displacement of primary hybridization partners by secondary targets measured in situ or post washing also deviate from expectations based on theoretical solution thermodynamics, but are consistent with predicted kinetic trends stemming from differences in either the toehold base length or branch migration.