The decomposition of three alkoxyacetic acids, methoxy, ethoxy, and isopropoxy acetic acids, has been studied by using ab initio calculations at the MP2/6-31G** level. Molecular mechanisms A and B have been characterized, corresponding to stepwise processes with formation of the corresponding alcohol and an alpha-lactone intermediate, achieved by the nucleophilic attack of either carbonylic or hydroxylic oxygen atoms followed by a ring opening process to yield formaldehyde and carbon monoxide. For ethoxyacetic and isopropoxyacetic acids, three additional reactive channels were thought to be possible on potential energy surface; mechanisms C and D are elimination processes to give the corresponding alkene and glycolic acid. The reaction pathways along mechanisms C and D take place by the transfer of a beta-hydrogen atom, with respect to the ether group, from the terminal methyl group to the hydroxylic or the alkoxylic oxygen atoms, respectively. The glycolic acid formed in the first step is then submitted to decomposition by water elimination and a-lactone intermediate formation. A final ring opening process yields formaldehyde and carbon monoxide. Molecular mechanism E is associated with a fragmentation process along a concerted one-step with concomitant formation of the corresponding alquene, formaldehyde, water, and carbon monoxide. The decomposition process is energetically favorable along the mechanism A, and the calculated rate coefficients agree with experimental data.