H-1 and C-13 NMR spectra were used to study environmental changes and surfactant packing transitions in acrylamide-water/AOT/toluene reverse microemulsions as the concentration of cosurfactant (acrylamide) was varied. These studies were motivated by the observation of cosurfactant-triggered threshold transitions in electron transfer for various reduction and oxidation reactions in these highly resistive microemulsions. The present study indicates that an interfacial packing transition of the surfactant (AOT, sodium bis-(2-ethylhexyl) sulfosuccinate) occurs at about the same cosurfactant concentration, 2-3% (w/w), as the threshold transition observed electrochemically and also provides a physical basis for a percolation threshold reported for these microemulsions.