Abstract: In order to ensure further Internet growth and to efficiently support quality-differentiated network services, users' choice options have to go beyond different service plans that only reflect a rough market segmentation. To further subdivide these segments, choice options also need to provide the means to let users express their current needs by instantaneously selecting a service quality. The theoretical argument is strongly supported by empirical evidence from the INternet Demand EXperiment (INDEX), a market trial for quality-differentiated Internet services. The investigation comprises the analysis of user heterogeneity, activity heterogeneity, and acceptance of QoS on demand.