DFT and ab initio calculations are performed to study the unimolecular decmposition pathways of malonic acid. The reaction is explored along three plausible decomposition pathways. The calculated results shows that decarboxylation is a two step process with an energy barrier of 32.16 kcal mol⁻¹ at the CCSD(T)/6-311++G(d,p) level which is in reasonable agreement with available data. In contrast to the usual interpretations, novel features of this study are dehydration and in situ decarboxylation–dehydration pathways that involve energy barriers of 67.13 and 44.75 kcal mol⁻¹, respectively at the CCSD(T)/6-311++G(d,p) level leading to the formation of carbon-suboxide and ketene along with conventional H₂O and CO₂. The thermal rate constants for the above decomposition pathways are also evaluated using Canonical Transition State Theory at 298 K.