We have studied the effect of adsorption of a low-density alkanethiol monolayer on the state of the Au(111) reconstruction. It is commonly believed that the substrate deconstructs following formation of a thiolate self-assembled monolayer, but our results suggest this is not always the case. Helium diffraction from 1-decanethiol and 1-octanethiol striped phase monolayers is exploited to establish the surface nearest-neighbor spacing and to illustrate a unit cell corresponding to the long dimension of the (23 x root3) reconstruction. Using our observed 0.198 Angstrom(-1) peak spacing and the (11.5 x root3) unit cell reported in the literature, we measure a substrate nearest-neighbor spacing of 2.76 A along the [1 (1) over bar0] direction, which represents the atomic spacing of the uniaxially compressed, reconstructed gold surface. Moreover, 1/2-order peaks in the diffraction from decanethiol/Au(111) demonstrate a distinction between neighboring thiolate dimers. These peaks are not observed for the octanethiol/Au(111) system. Therefore, the 1/2-order peaks are not an inherent characteristic of alkanethiol SAMs. The most likely explanation for these peaks is a reconstructed substrate. Complementary scanning tunneling microscopy data are also presented that show persistence of the reconstruction during growth of a decanethiol striped phase monolayer and no evidence for vacancy islands typically associated with deconstruction or alternate reconstructions. Our model involving a still-reconstructed substrate is consistent with all of the available data, while alternative models fail to explain the results presented in this article.