A convenient, stepwise route to diphosphasilacyclopropanes starting from appropriate substituted P,P'-bis-(fluorosilyl)diphosphanes is reported, The reaction of Is(2)Si(F))-PH-PH-Si(F)Is(2) (IS = 2,4,6-trusopropylphenyl) (1), which exists as a 1:2 mixture of stereoisomers (d,l-racemate and meso form), with I equiv of ''BuLi in THF furnishes the lithium phosphanide Is(2)Si(F)-P[Li(THF)(n)]-PH-Si(F)Is(2) (5), which is converted by thermolysis and LiF/THF elimination into the corresponding diphosphasilacyclopropane Is(2)Si-PH-P-S(F))Is(2) (2a), The unusually low s-contribution of the Si-P/P-P bonds and ring strain of the SiP2 skeleton in 2a is reflected by its low values of the (1)J(P-H) (147 Hz) and (1)J(P,P) coupling constants (0 Hz!). Compound Za consists of an almost regular SiP2 triangle as shown by single-crystal X-ray analysis. Lithiation of the P-H function in Za in the presence of THP; leads to the corresponding P-Li(THF)(3) derivative 2b which has also been characterized by X-ray analysis. The same product is accessible by double lithiation of 1 with 2 equiv of (BuLi)-Bu-n in THF. The SiP2 three-membered ring is isosceles, having different Si-P distances (2.252(3) and 2.171(3) Angstrom), and the P atoms an pyramidally coordinated. Heating of 2b liberates LiF/THF and furnishes the desired P2Si2-bicyclo[1.1.0]butane (Is)(2)Si-P-P-Si(Is)(2) (3), which shows a characteristic high-field resonance signal at delta = -300.2 and Si-29 satellites ((1)J(P, Si) = 83 Hz). Ab initio calculations at the MP2/6-31G* level have been performed in order to investigate the stability of the bicyclo[1.1.0]butane-like isomers of H4P2Si2 versus their respective buta-1,3-diene-like pendants.