The first phosphorus arsenic polycycle t-Bu 6 As 2 P 6 (1) was synthesized by dehalogenation of a 3:1 mixture of tert-butyldichlorophosphane and phosphorus trichloride using magnesium. 1 has been characterized by spectroscopic methods and by a crystal structure analysis having the t-Bu 6 Ps-analogous 2,2′,3,3′,4,4′-hexa-tert-butyl-l′-diarsa-2,2′,3,3′,4,4′-hexaphospha- bicyclobutane structure. In solution, the rotation around the exocyclic As-As bond is restricted already at room temperature. 1 crystallizes triclinically in the space group PT (No. 2) with a = 919.4 pm, b = 924.3 pm, c = 1171.7 pm, a = 97.21°, β = 102.98°, γ = 112.75°, and Z = 1 formula unit (V = 0.8694 nm 3 , D x = 1.295 Mg · m -3 ). The crystal structure has been solved by 'direct methods' and refined to R = 0.048 with 2131 observed reflections. The molecule with symmetry C 1 consists of two trans oriented and slightly folded four-membered P 3 As rings. The exocyclic bond length (d(As-As) = 244.1 pm) as well as the average endocyclic bond lengths d̅(As-P) = 235.6 pm; 3d̅(P-P) = 222.6 pm) are normal. The arrangement of the tert-butyl substituents with average bond lengths d̅(P-C) = 189.9 pm and d̅(C-C) = 1 5 2 . 5 p m is all-trans.