A calorimetric study of methabenzthiazuron (MET) has provided evidence of chemical transformation in the solid state at room temperature. Traces of water vapour were absorbed to form a hydrate, which produces a binary eutectic system with MBT at a temperature of 380.3 K and an estimated mole fraction of MBT of x(A) = 0.78. The mean value of the hydration rate of MBT at room temperature is ($) over bar V-A-->B = 3.7 x 10(-5) mol% min(-1). The mean value of the thermal decomposition rate of MBT at the melting temperature T-m degrees = 393.5 K is ($) over bar V-A = 0.01% mol min(-1). The highest rate of decomposition, ($) over bar V-E = 0.14% mol min(-1), was observed at the eutectic temperature.