AbstractA new synthesis of cyclobutenedione(1)is described. It is found that the tricyclic cyclobutanedione3splits into1and cyclopentadiene if subjected to flash pyrolysis conditions. The dienophilic reactivity of 1 allows the construction of various polycyclic cyclobutanediones, such as3–5and12–14.Other new polycyclic diones, like9–11, 15and16are prepared by means of the acyloin condensation. The low stereoselectivity of1with cyclopentadiene is compared with the selectivity of other cyclobutenes, and the results are placed in the context of secondary interactions and steric hindrance. The unusual colour properties of these diones, from colourless1to blue6are investigated by means of photoelectron spectroscopic measurements and semi‐empirical calculations (MNDO, HAM/3, CND