The properties of substituted cyclobutene-1,2-diones 1 are examined by the use of O-17 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 O-17 chemical shifts reproduce the measured trends. The dianions of squaric and deltic acids are highly stabilized by negative charge delocalization to the oxygens.