Are porins on the inner or on the outer membrane of mitochondria?
This is not my field, but The Transporter Classification Database would appear to be a reliable source and states that:


  The best characterized members of the MPP family are the voltage-dependent anion-selective channel (VDAC) porins in the mitochondrial outer membrane.


Searching through the Protein Data Bank one can find a crystal structure for human voltage-dependent anion channel 1 and the associated paper also states that it is in the outer mitochondrial membrane, so you would imagine that they should know.