The adsorption of CN- on gold single crystal has been monitored by visible-infrared difference frequency generation (DFG). We show a significant effect of the surface orientation on the adsorption strength of CN-, which decreases from (110) to (100) and (111). Both the linewidth and the potential tuning rate of the DFG resonances do not depend on the electrode surface orientation, indicating that CN- is tightly and linearly bound to its nearest surface gold atom.