The structures ofcis-2-(m-nitrophenyl)cyclopropanecarboxylic acid (1) and 1-phenyl-2-methylcyclopropene-3-carboxylic acid (2) have been determined by X-ray methods. Crystals of1are triclinic, space group, witha = 7.422,b = 8.615,c = 9.499 Å, α = 60.24, β = 63.76, γ = 71.36°, and two molecules in the unit cell. Crystals of2are monoclinic, space groupP21/c, with four molecules in a unit cell of dimensionsa = 9.345,b = 13.286,c = 8.187 Å, and β = 98.22°. The carboxyl group of1approaches the bisecting conformation. The phenyl group is 37° from this conformation, by contrast with the unhindered compound 2-(p-nitrophenyl) cyclopropyl methyl ketone. In2the carboxyl group bisects the three-membered ring, indicating that carbonyl-cyclopropene π interactions can occur if sterically allowed. The two rings are essentially coplanar, permitting maximum interaction of the phenyl group and the double bond. In both compounds, the carbonyl oxygen of the carboxyl is the nearer oxygen to the three-membered ring.