The rate of degradation of cyromazine was examined under laboratory conditions in a number of experiments in various soils. In the laboratory studies cyromazine was degraded with a DT50 of between 2.7 and 56 days, a median value 26.6 days and a realistic worst case of 51.5 days (90th percentile). These values will be the basis for calculation of Predicted Environmental Concentrations (PECs) in soil resulting from the use of cyromazine formulations in Europe. Rapid degradation of cyromazine with half-lives between 2.7 and 4 days is observed in studies performed with soils with high microbial activity (Plücken, 1986, Glänzel 2000). In three Dutch soils (Deneer 2003) cyromazine was found to degrade slower with half-lives ranging from 15 to 56 days with the lowest DT50 value for the soil with the highest micorbial activity. Biological degradation is hence considered to represent the main dissipation process for cyromazine in soil. The microbial activity is therefore assumed to be the main contributing factor to the dissipation rate. In spite of high microbial activity in the acidic UK soil 18 acres (Adam 2003) a DT50 value of 50 days was observed. Besides microbial activity, the difference of the behaviour of cyromazine in the various soils can be explained by a difference in bioavailability depending on the soil adsorption as a function of protonation and soil pH. The pKa value of cyromazine is 5.2 indicating a weak tendency for cation binding to more acidic soils and consequently lower availability for biodegradation. A dependence of the degradation of cyromazine on the temperature conditions of the study is observed. A decrease in the temperature from 20°C to 10°C results in an increase of the half-life of about a factor of two. No clear correlation between the soil characteristics (e.g. clay content) and the degradation rate of cyromazine was found. Under anaerobic conditions, degradation of the parent compound continues but it proceeds at a lower rate. Melamine is the major soil metabolite of cyromazine. The amount of melamine observed depends on the ratio of its rate of formation to its rate of degradation and is thus variable. Laboratory studies revealed peak values of melamine between 31 and 75% of applied radioactivity occurring between 19 and 120 days of incubation. The dissipation half-life of the main metabolite melamine was determined in six cyromazine laboratory studies where the degradation of melamine was calculated taking formation and decline into account. Additionally, three studies were performed with melamine directly applied to soil. The resulting half-lives range from 88 to 217 days with a mean value of 154 days and a realistic worst case (90th percentile) of 199 days.