At temperatures between 1150 and 2000 K and pressures between 0.1 and 0.2 MPa, the thermal decomposition of carbon suboxide (C3O2) behind reflected shock waves was investigated with a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) connected to the end flange of a shock tube enabling rapid repetitive (100 kHz) measurements of the gas-phase composition. Concentration–time profiles for C3O2 and CO were measured and compared to simulations based on an improved mechanism for C3O2 decomposition and carbon cluster growth. In addition, relative concentrations of C atoms and C2 molecules were detected and related to model predictions. For temperatures up to 1800 K, satisfactory agreement between experimental data and calculations was obtained. At higher temperatures, measurements and simulations differed noticeably. The importance of C2 for the growth of carbon clusters was confirmed.