Microdroplets in microfluidic systems can be used as independent microreactors to perform a range of chemical and biological reactions. However, in order to add new reagents to pre-formed droplets at defined times, to start, modify, or terminate a reaction, it is necessary to perform a controlled fusion with a second droplet. We describe and characterize a simple and extremely reliable technique for the one-to-one fusion of droplet pairs in a microfluidic system at kHz frequencies. The technique does not require special channel treatment, electrical fields or lasers to induce droplet fusion. Instead, we make use of transient states in the stabilization of the droplet interface by surfactant, coupled to a proper geometrical design of a coalescence module, to induce the selective fusion of a droplet stabilized by surfactant (re-injected) with a droplet which is not fully stabilized (generated on-chip). Using a 1.2-fold excess of the surfactant-stabilized droplets ∼99% of the partially stabilized droplets were fused one-to-one with surfactant-stabilized droplets. Even when the surfactant-stablized droplets were in 5-fold excess, over 96% of the partially stabilized droplets were fused one-to-one. The fused droplet contains enough surfactant to inhibit further fusion events. After fusion, the droplets were fully stabilized by additional surfactant provided in the carrier oil, which allowed the fused droplets to be collected, incubated off-chip and re-injected onto a microfluidic device without any undesired coalescence.