The objective of this research was to develop a mathematical model that simulates and predicts chemical and biological fundamentals of nitrification in pilot-scale distribution systems and to evaluate kinetic parameters associated with the fundamentals. Six pilot-scale distribution systems were set up at the Water Science and Engineering Laboratory on the campus of the University of Wisconsin-Madison. Water from Lake Mendota was used as the source water to the distribution systems after being treated with conventional alum coagulation and filtration processes. Prior to entering the distribution systems, the treated Lake Mendota water was chloraminated so that the chloramine residual at the end of the system was in a range of typical levels in full-scale distribution systems (i.e., 0.5~2.5 mg Cl2/L). The free ammonia dose was also controlled at the entrance of the pilot-scale distribution system. Each of the six pilot-scale distribution systems was set up with two connected tanks designed to behave as two completely-mixed-flow-through reactors. In total, the pilot system was comprised of twelve tanks. Each tank had a unique combination of controlled conditions including chloramine residual, influent free ammonia concentration, and detention time. Chloramine residuals at the end of each system could be controlled at 0.5 to 1.0 mg Cl₂/L or 2.0 to 2.5 mg Cl₂/L. Influent excess ammonia concentrations could be controlled at high (Cl_:NH₃=3:1), medium (Cl₂:NH₃=4:1), or low levels (Cl₂:NH₃=5:1). The target detention time was 1 day or 3 days. Each tank was connected to a PVC pipe loop with 1.27 cm diameter and 7.6 m length. Recycle flow rate in the pipe loop was maintained to make each tank a completely mixed reactor and, at the same time, could simulate typical low-flow conditions (e.g. 0.07 m/s, Boe-Hansen et al., 2002) in real distribution systems. Samples were collected regularly at the 1-day tank influent, 1-day tank effluent (same as influent to 3-day tanks), and 3-day tank effluent. Sampling data included concentrations of chloramines, free ammonia, dissolved oxygen, nitrite, nitrate, HPC, and AOB in the bulk water. Ambient conditions temperature and pH were also monitored. Includes 14 references, figure.