Droplet microfluidics has found many applications in chemical reactions. In most of the studies, however, reactions are performed in droplet-based reactors either through introducing various reagent streams into a single droplet, or upon coalescence or merging of two droplets encapsulating different reagents. Here we introduce a facile microfluiclic mass-transfer control approach for synthesizing titania microparticles using droplets as microreactors. Droplets of a water soluble and stable titania precursor aqueous solution, titanium (IV) bis(ammonium lactato)-dihydroxide (TiBALDH), were dispersed in an oil continuous phase containing a small amount of ethanol, which can precipitate TiBALDH and produce titania. Precipitation was initiated and controlled by the mass-transfer of ethanol from the continuous phase into the droplets. Titania particles with different degrees of compactness were formed and well encapsulated inside the droplets by varying the concentrations of TiBALDH in the droplet phase and ethanol in the continuous phase. This paper provides a new approach for synthesizing nanoparticles or microparticles in droplet microfluidics based on a mass-transfer control approach. (C) 2014 Elsevier Ltd. All rights reserved.