A version of the local state isomorphism theorem is used to show how n
onlinear model structures, within some specified class, can be tested
for indistinguishability from some given structure via a suitable choi
ce of their respective parameters. This theory is applied to a two-com
partmental structure with an assumed Michaelis-Menten elimination from
compartment 1. Models tested for indistinguishability include structu
res with Michaelis-Menten elimination from compartment 2 in addition t
o, or instead of, that from compartment 1.