Changes in local lipid composition are thought to play a key role in regulating many complex cellular processes. By studying lipid organization in artificial lipid bilayers the physical principles underlying these process can be studied in detail. However, such in vitro measurements are often hindered by heterogeneities in the lipid composition of individual bilayers prepared by current bulk methods. Here, the lipid composition of an individual droplet interface bilayer is varied by lipid titration into the bilayer from the oil phase in a microfluidic device. Control of lipid composition allows the reversible switching between single- and two-phase regions and sampling of specific lipid compositions in an individual bilayer. This method enables controlled modulation of composition-sensitive processes in a single lipid membrane.