CH) 3 P 3 structures have been investigated by ab initio favored for the [4+2] cycloadditions of HCP to 1,2-and 1,3diphosphete (to give triphospha Dewar benzenes); the (MP4SDTQ/6-31+G**//MP2(fu)/6-31G*) and DFT (B3LYP/6-311+G**) methods. The framework is the main factor computed activation barriers are low: ∆H* 298 = 2.2 and 0.8 kcal mol -1 , respectively, at MP4/6-31+G**//MP2(fu)/6-31G* determining the realtive energy of isomers; the substitution pattern is less important. The thermodynamic stability order (2.0 and 1.5 kcal mol -1 at MC-QDPT2/6-31G*//CAS-SCF(6,6)/6-31G*). Mono-as well as ortho-, meta-, and para-of heteroatom-substituted benzenes Ͼ benzvalenes Ͼ Dewar benzenes Ͼ prismanes Ͼ bicyclopropenyls parallels the diphosphabenzenes and the triphosphabenzenes are found to be as aromatic as benzene according to geometric and situation in the (CH) 6 hydrocarbons. The 3 HCP Ǟ 1,2,3triphosphabenzene trimerization energy is only -84 kcal mol -1 ; energetic criteria. NICS calculations (a magnetic criterion) suggest only slightly reduced aromaticity. half as large as that for the 3 acetylene Ǟ benzene conversion (-164 kcal mol -1 ). Head-to-tail additions are ments are involved in the few examples of RCP trimeriz-