The title compound, [Ru(C(10)H(15))(C(10)H(12)O)][B(C(6)H(5))(4)], crystallizes as discrete (η(5)-penta-methyl-cyclo-penta-dien-yl)Ru(η(6)-4-phenyl-butan-2-one)](+) cations and [BPh(4)](-) anions. In the cation, the non-H atoms of the butan-2-one group are approximately planar (r.m.s. deviation =