The gas-phase isomerization and decomposition reactions of dimethyl ethylphosphonate (DMEP) are investigated using the CBS-QB3 method followed by the calculation of rate constant for all reaction pathways using Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Three conformational isomers C1, C2, and C3 are identified for DMEP having stability order C1 (0.00) < C2 (0.11) < C3 (1.87). Each conformer can isomerize via H-transfer reaction and can decompose via CH3OH, CH2, and H-2 elimination at higher temperatures. The conformers C1, C2, and C3 can isomerize to IM1, IM2, and IM3, respectively, via different pathways. Decomposition requires much higher activation energy and therefore higher temperatures than the corresponding isomerization. For instance, the most stable conformer C1 isomerizes to IM1 twice as fast as decomposing to P1 + CH2 at 1000 K whereas the least stable conformer C3 isomerizes to IM3 10(4) times faster than decomposing to P5 + CH3OH. Only one decomposition channel is identified for C1 and two different decomposition channels are identified for C2 as well as for C3. The decompositions of C2 and C3 to P2 + CH3OH and P5 + CH3OH, respectively, are predicted to be more favorable thermodynamically as well as kinetically over the other decomposition channels within the temperature range 1000-3000 K. For the lack of experimental data, we have calculated the low as well as high-pressure limit rate constants for the decomposition reactions of DMEP. In addition, consistent and reliable enthalpies of formation at 298.15 K (Delta H-f degrees(298.15)) have been computed for all the species involved in the isomerization and decomposition reaction of DMEP. The results obtained for DMEP for the reaction mechanism and energetics are compared with that for DMHP and DMMP.