Crystals are monoclinic, space group P2,/c, with Z = 4, in a cell of dimensions : a = 6.39. b = 10.98, c = 9.76 8, p = 105.6". The structure w a s solved by direct phase-determining methods and refined by full-matrix leastsquares calculations to R 0-085 over 638 reflections. The crystals contain centrosymmetric hydrogen-bonded dimers with a conformation in which the C=O bond points over the cyclopropane ring.DURING their investigations on the mechanism of cyclopropane formation from epoxides via the Wittig reaction, Ghirardelli and Izydore obtained mixtures of isomeric 2,3-dimethoxycyclopropanecarboxylic acids, (1)-( 111) , from reactions involving ( + ) -(2R,3R) -2,3-epoxybutane and cis-2,3-epoxybutane. I n order that an analysis of the reaction mechanism could be made, it was imperative that the identity of each isomer be established unequivocally. The constitution of (I) was inferred from its optical activity. One of the other two isomers was isolated from the product mixture as a solid, 1ii.p. 79--80.5", and we have subjected it to single-crystal R. G. Ghirardelli and R. ,4. Izydore, personal communication.X-ray analysis to determine its configuration We here report the results of this analysis. EXPERIMENTAL Crystal Data.-C,H,,O,, n1.p. 79-80-5", M = 114.1. Monoclinic, a = 6.39(1), b = 10-98(2), c = 9.76(2) A, 1.148, F(000) = 248. Cu-K, radiation A = 1.542 -4, p(Cu-K,) = 7.1 cm-l. Space group P2Jc (C&J from systematic absences: OkO when k # 2n and h0Z when 1 # 2n.Crystallografihic Measurements.-The crystals have a tendency t o sublime at room temperature, and so those used for the data collection were sealed in thin-walled glass capillaries. Unit-cell dimensions were evaluated from