Isotopic substitution has been used in conjunction with neutron reflection to make a selective study of the counterion layer beneath a charged insoluble monolayer spread on water. The insoluble monolayer consisted of a mixture of sodium docosyl (C-22) sulfonate and docosanol, and its charge was adjusted by varying the relative amounts of anionic and neutral species in the layer. The counterion was the tetramethylammonium ion, TMA(+). The counterion distribution was determined at three surface charge densities, corresponding to areas per negative charge of about 30, 60, and 85 Angstrom(2), and at three bulk electrolyte (TMACl) concentrations, 0.1, 0.01, and 0.001 M. There was found to be significant penetration of the counterions into the headgroup region of the amphiphile to form a well defined "Stern" layer, and there was also a thicker layer corresponding to part of the diffuse layer. The changes in the behavior of the counterions with changes in surface charge density and bulk electrolyte concentration were qualitatively consistent with double-layer theory. However, a complication in applying theory to the diffuse part of the double layer was that not all the counterions could be observed by the neutrons. This may have been because of preferential adsorption of Na+ present in the original spreading solution or because of roughness of the counterion distribution. The obervation by neutron reflection of penetration of the amphiphilic layer by the counterions was consistent with the unusual behavior of the pi-A isotherms on addition of TMACl.