Structural studies of thermal decomposition products of trimethylamine have been performed by gas electron diffraction aided by mass spectrometry. Trimethylamine vapor is heated in a quartz tube shortly before the nozzle tip which is kept at room temperature. When the temperature inside the quartz tube is 515°C, N(CH 3 ) 3 simply decomposes into CH 4 and short-lived CH 3 - CH 2 . However, more complicated reactions, which include CH 3 -N CH 2 → CH 4 + HCN, take place at the reaction temperature of 535°C. The zero-point average structure of N-methylmethyleneimine (CH 3 -N CH 2 ) has been determined by a joint analysis of electron diffraction data and rotational constants, and then compared with ab initio calculations. Principal structural parameters are as follows:r z (N C) = 1.279(6) , r z (N-C) = 1.458(8) and θ z (CNC) = 116.62(12)°. The molecular structure of trimethylamine has been determined much more precisely than that given in the literature.