Investigating the role of intestinal microbial populations significantly relies on the assumption of stability. Therefore, the microbial community composition of the simulator of the human intestinal microbial ecosystem was qualitatively, quantitatively and functionally characterised during reactor start-up to evaluate its capacity to produce a stable bacterial community, representative for the human intestinal tract. Using moving window correlation, a stability criterion was introduced to analyse the stability over time of the PCR-DGGE, plate counts, short chain fatty acids and ammonium results. A community was regarded stable when minimum 80% correlation was measured over at least one cell residence time. Species composition stability was reached after about 2 weeks, while it took some 3 weeks to reach functional stability. The combination of PCR-DGGE with moving window correlation proved to be an efficient approach to quantitatively evaluate the stability of the in vitro cultured intestinal microbial community.