A phylogenetic analysis of monocots and related dicots was conducted, using a four-gene matrix consisting of two genes from the plastid genome (matK and rbcL) and two from the mitochondrial genome (atpA/atp1 and cob). The taxon sample includes 101 monocots and 36 dicots, and all four genes were sampled for all 137 taxa. Jackknife support was assessed for clades resolved by the four-gene analysis, and compared to support for the same clades by each of the four three-gene subset matrices, in order to quantify the degree to which each gene contributed to or detracted from support for each clade. Instances of positively and negatively correlated support for clades by genes of the same and different genomes were observed. In particular, the placement of Acorus within a clade that also includes Tofieldiaceae, Araceae, and Alismatales s.s., as opposed to its frequent placement as sister of all other monocots, is supported by atpA and matK. The results indicate that genes from the mitochondrial genome provide a unique test of relationships that have been inferred with plastid-encoded genes.