Detailed investigations have been conducted on a set of four pilot scale subsurface flow (SSF) constructed wetlands in order to characterise heat transfer, mass dispersion and biological performance mechanisms. These studies have followed the beds from post construction through unplanted hydraulic base line studies to the current status of mature stands ofPhragmites australis. Experimental observations indicate that in unplanted beds, daily thermal fluctuations are depth dependent and range from 1 to 9 degrees Celsius. These fluctuations result in daily thermal inversions, and enhanced mixing and oxygen transport. For planted beds, thermal fluctuations are depth independent, and have a constant amplitude of 2 degrees Celsius. Planted beds may be thermally stratified. Lithium tracer studies corroborate these results for the planted bed. In addition, performance studies indicate that organic pollutant removal is probably limited to organic suspended solids removal, with subsequent biological breakdown. Current first‐order plug flow models can not account for these operational issues. A combined model is necessary to account for lateral dispersion, temperature gradients and settling of suspended solids to accurately reflect real biological removal mechanisms.