In this paper, we studied effects of phosphonium dications P2C5 and P2C10 on bilayer planar phospholipid membrane (BLM) and rat liver mitochondria. In line with our previous observations [M.F. Ross, T. Da Ros, F.H. Blaikie, T.A. Prime, C.M. Porteous, I.I. Severina, V.P. Skulachev, H.G. Kjaergaard, R.A. Smith, M.P. Murphy, Accumulation of lipophilic dications by mitochondria and cells, Biochem. J. 400 (2006) 199-208], we showed both P2C5 and P2C10 are cationic penetrants for BLM. They generated transmembrane diffusion potential (Delta Psi), the compartment with a lower dication concentration positive. However, the Delta Psi values measured proved to be lower that the Nernstian. This fact could be explained by rather low BLM conductance for the cations at their small concentrations and by induction of some BLM damage at their large concentrations. The damage in question consisted in appearance of non-Ohmic current/voltage relationships which increased in time. Such a non-Ohmicity was especially strong at Delta Psi >100 mV. Addition of penetrating lipophilic anion TPB, which increases the BLM conductance for lipophilic cations, yielded the Nernstian Delta Psi, i.e. 30 mV per ten-fold dication gradient. In the State 4 mitochondria, dications stimulated respiration and lowered Delta Psi. Moreover, they inhibited the State 3 respiration with succinate or glutamate and malate (but not with TMPD and ascorbate) in an uncoupler-sensitive fashion. Effect on the in State 4 mitochondria, similarly to that on BLM, was accounted for by a time-dependent membrane damage. On the other hand, the State 3 effect was most probably due to inhibition of the respiratory chain Complex I and/or Complex III. The damaging and inhibitory activities of lipophilic dications should be taken into account when one considers a possibility to use them as a vehicle to target antioxidants or other compounds to mitochondria.