Pentacarbon dioxide, officially penta-1,2,3,4-tetraene-1,5-dione, is an oxide of carbon (an oxocarbon) with formula C<sub>5</sub>O<sub>2</sub> or O=C=C=C=C=C=O.
The compound was described in 1988 by Günter Maier and others, who obtained it by pyrolysis of 2,4,6-tris(diazo)cyclohexane-1,3,5-trione (C<sub>6</sub>N<sub>6</sub>O<sub>3</sub>). Diazo transfer can produce the latter compound from phloroglucinol. It is stable at room temperature in solution. The pure compound is stable up to âÂÂ90 ðC, at which point it polymerizes.