We report on depolymerization reactions of poly(phosphinoboranes). The cleavage of the polymers [H₂PBH₂]_n (2 a), [tBuHPBH₂]_n (2 c), [PhHPBH₂]_n (2 e) and the oligomer [Ph₂PBH₂]_n (2 b), with strong Lewis bases (LBs), in particular with NHCs, leads to the corresponding monomeric phosphanylboranes R¹R²PBH₂LB. It is observed that the depolymerization depends on the strength and stability of the LBs as well as on the substitution pattern of the poly(phosphinoboranes). The solid state structures of the monomeric phosphinoboranes H₂PBH₂NHC^Me (NHC=N-heterocyclic carbene) (4 a), H₂PBH₂NHC^dipp (5 a) and tBuHPBH₂NHC^Me (4 c) were determined. DFT calculations support the experimentally observed reaction behavior.