Both cation and anion in the title compound, C2H12BN2+.I-3(-), lie on a crystallographic mirror plane and are bound in the lattice by N-HcI(-) hydrogen bonds, forming layers. Methyl-H-borane-H dihydride [-C-H(delta(+))c(delta(-))H-B-] inter-actions between molecules crosslink adjacent layers, giving `sandwich' stacking along the a axis.