Reflectance absorbance infrared (RAIRS) and thermal desorption (TPD) data for methyl formate and ethyl formate chemisorbed on Ni(111) are found to be consistent with surface coverage dependent rotational isomerization. The most stable gas phase rotameric forms of these molecules are the (Z) or cis forms. However, the reflectance infrared data provide direct evidence for isomerization to the (E) or anti forms as their surface coverages are increased. This rotational isomerization, around the C-O bond, is observed to occur at coverages less than half that required to saturate the chemisorbed layers. In combination with infrared integrated intensity data, the thermal desorption results indicate that steric repulsion is responsible for the observed rotational isomerization. It is argued that two factors contribute to a reduction of the rotational barrier. First, steric repulsion is reduced continuously along the (Z) to (E) rotational coordinate. Second, image dipole stabilization increases continuously along the (Z) to (E) rotational coordinate. A number of interesting parallels exist between the chemisorption properties of these three ester molecules on Ni(111) and the properties of Lewis acid-carbonyl molecule adducts. These similarities will be discussed briefly with reference to use of Lewis acids to achieve stereoselective control in organic synthesis, and with reference to the issue of enantioselective heterogeneous catalysis.