Thermal electron attachment to C3O2 has been studied by the pulse radiolysis–microwave cavity technique. The time variation of electron concentration for mixtures C3O2 with Ar and N2 shows decays with two components and the profile changes significantly depending on both C3O2 and buffer-gas pressures. This can be interpreted by a fast electron attachment to C3O2 to form a compound negative ion in equilibrium with the free electron followed by a slow stabilization of the ion leading to a stable ion. Simulations for the observed decay profiles indicate the presence of two electron attachment–detachment equilibria, one associated with electron and C3O2 and the other including electron, C3O2, and the buffer-gas molecule. The slow stabilization occurs by two-step three-body processes involving intermediate dimer ion. This is supported by a ‘‘negative temperature dependence’’ observed for the stabilization rates between 270 and 325 K. Though the electron affinity of C3O2 is negative, that of (C3O2)2 is likely to be positive. The role of electron attachment has been discussed in relation to an anomalously large cross section for ionization of a high-Rydberg atom by C3O2 reported previously.