The dynamics of the two unimolecular reactions that initiate the thermal decomposition of methyl nitrite were investigated by classical trajectories and statistical variational efficient microcanonical sampling-transition state theory. These two channels are (I) O-N bond dissociation to produce CH3O and NO and (II) concerted elimination through a four-center transition state to form CH2O and HNO. In order to perform both types of calculations, a potential energy function was developed, which reproduces reasonably well the energies, geometries, and frequencies selected from the literature. Microcanonical rate coefficients and branching ratios were obtained by each method at total energies ranging from 100 to 240 kcal/mol. The computed branching ratios indicated that reaction I is markedly faster than reaction II, which agrees with the experimental observations. It was found that for energies up to 160 kcal/mol. the dynamics of reaction I is intrinsically Rice-Ramsperger-Kassel-Marcus (RRKM), but for the highest energies the behavior becomes intrinsically non-RRKM. The classical trajectories showed that the elimination process takes place via a regular dynamics during the last moments before reaction, which is clear evidence for nonstatistical behavior. Analysis of the trajectory rates computed for the deuterated species revealed that the dissociation process exhibits an inverse secondary isotope effect.