With health concerns developing about the use of glyphosate (phosphonomethylglycine, PMG), the world's most used herbicide, the possibility of destruction of stockpiles via incineration arises. Little is known, however, about the possible toxic products of decomposition. We have performed a quantum chemical computation of the mechanism of thermal decomposition of PMG. Two initiation channels, one producing sarcosine and the other producing N-methylaminomethylphosphonic acid, have been located. Both the initial products decompose to dimethylamine (DMA), and the mechanism of further decomposition and toxic products is explored. Global potential energy surfaces for the initial decomposition of DMA are presented together with chemical kinetic modeling wherein the rate constants employed have been calculated from the quantum chemical data. Time and temperature evolution of the expected toxic products are presented and discussed.