Diisopropyl methylphosphonate (DIMP) is a chemical weapon agent surrogate. Quantifying the rates of thermal decomposition of such compounds is important to enable predictions of spread of respective toxic vapors in different scenarios. An experimental setup is designed and built to quantify the rate of decomposition of DIMP at temperatures of 200–350° C. Liquid DIMP is fed from a brass evaporator heated to 140° C and vented with argon. It is mixed with a pre-heated carrier gas (air or nitrogen) in a steel flow reactor. The gas is sampled from the reactor after a certain residence time and sent to a Fourier transform infrared (FTIR) analyzer. Under 350° C, the thermal decomposition of DIMP in air occurs much faster than in nitrogen. In air, the decomposition of DIMP can be described as the first order reaction with the rate constant k T s-1= 10 7.4±2.5· exp-21.4±6.6 kcal/m o l RT. Despite measuring substantial reduction in the DIMP concentration due to its thermal decomposition, present measurements could not detect the presence of propene and other decomposition products.