The technique of isotopic substitution in neutron diffraction has been used to measure the structure of saturated solutions of lithium and potassium in ammonia. Isotopic substitution of *N by N-15, combined with difference analysis, shows that K+ acts as a "structure breaking" ion while Li+ acts as a "structure making" ion. As a consequence, it is only for the latter that we observe intermediate range order, in the form of a pre-peak at similar to 1 Angstrom(-1) in the k-space data. From our analysis, we also find that lithium is tetrahedrally coordinated by ammonia molecules, at both 230 and 100 K. Potassium, on the other hand, is octahedrally coordinated at 160 K. (C) 2000 American Institute of Physics. [S0021-9606(00)50716-8].