Structural characteristics and order within n-alkanethiols, alpha,omega-alkanedithiols and alpha-cycloalkyl-omega-alkanethiols monolayers self-assembled on gold have been investigated using high resolution electron energy loss spectroscopy (HREELS). The coherent domain sizes have been estimated from the angular distribution of the elastic peak as a function of the nature of the alkane chain and of the immersion time in the thiol solutions. These data have revealed many defects in the organization of the monolayers on evaporated gold substrates. It has been shown that the domains are smaller on Au(100) than on Au(111).