It has been shown by experimental and theoretical studies that exchange interactions in non planar pi systems are strongly dependent on the geometrical parameters characterizing the non planarity. A study of the exchange interaction in isolated nitroxide biradicals is presented for non planar paraphenylene bis nitroxide 3 [1,4-di-t-butyl-2,5-bis(t-butylaminoxyl) benzene (anti)], non planar metaphenylene bis nitroxides 7 and planar metaphenylene bis nitroxide 9. Determination of positive or weak negative exchange interactions in isolated molecules in dilute solution is difficult and can be inaccurate. 3 is a ground state singlet 2J/k = -35K. Metaphenylene bis nitroxide 7g [1-t-butyl,3,5-bis(t-butylaminoxyl) benzene] and 9 [1,2,9-tetrahydro 2,2,4,4,6,6,8,8-octamethyl-4H,6H-benzo[1,2d; 4,5d’]bis(1,3 oxazine)-1,9 dioxyl] are probably ground state singlets, the triplet-singlet separation being smaller than 10K, contrary to theoretical predictions. X-Ray structure of 7a A [1,3,5-trimethyl-2,6-bis(t-butyl aminoxyl)benzene (syn)], 7g and 9 are reported. In pure crystalline solid, however, these three biradicals have a susceptibility characteristic of triplet radical-pairs in weak antiferromagnetic interaction with their neighbours. The problems (including the transferability of the exchange interaction) raised by these apparently contradictory results are discussed.