The properties of substituted cyclobutene-1,2-diones 1 are examined by the use of (17)O NMR spectroscopy and theoretical calculations and compared to those of cyclopropenones 2 and other models. Cyclobutene-1,2-diones have less negative charge per oxygen compared to cyclopropenones, and electron donation by substituents enhances the negative charge on oxygen. Calculated (17)O chemical shifts reproduce the measured trends. The dianions of squaric and deltic acids are highly stabilized by negative charge delocalization to the oxygens.