The phase behavior, conductance, and viscous behavior of nonaqueous microemulsions formed by the combination of the solvents [formamide (FA), ethylene glycol (EG), propylene glycol (PG), dimethylformamide (DMF), and dimethylacetamide (DMA)] and oils [heptane (Hp), octane (Oc), isooctane (i-Oc), xylene (Xy), and toluene (TI)] in presence of aerosol OT (AOT, sodium 1,4-bis(2-ethylhexyl)sulfosuccinate) have been studied. The ternary phase diagrams of the nonaqueous solvent systems with i-Oc are more or less similar; the single phase areas have fairly large viscous zones toward the amphiphile end, that of FA/AOT/i-Oc being significant. Both viscosity and conductance have demonstrated percolation and internal structure formation. The intrinsic viscosity and Huggin’s constant at constant solvent/AOT mole ratio (omega) for a number of systems have supported spherical (or minorly ellipsoidal) nonsolvated dispersions. Except FA, all the other solvents obeyed the viscosity equations of Vand, Moulik, and Eiler. The thermodynamics of a solution of the nonaqueous solvents in AOT/i-Oc medium resulting Winsor IV microemulsification has been calorimetrically studied. The FA/AOT/i-Oc system showed exothermicity, whereas the systems of EG, PG, DMF, and DMA with i-Oc and AOT exhibited endothermicity. The enthalpies of solution are significantly low and the specific heats of the resultant mixtures are very close to one another.