Experimental and computational investigation is carried out to elucidate the influence of stoichiometric mixture fraction, ζst, on the structure and critical conditions of extinction of nonpremixed dimethyl ether (DME) flames. The stoichiometric mixture fraction represents the location of a thin reaction zone in terms of a conserved scalar quantity. The counterflow configuration is employed, wherein two reactant streams flow towards a stagnation plane. One stream is made up of DME and nitrogen (N2) and the other stream is oxygen and N2. Previous studies have shown that critical conditions of extinction depend on (Formula presented.) and the adiabatic temperature (Formula presented.). Therefore, the present investigation is carried out with the composition of the reactants in the counterflowing streams so chosen that the adiabatic temperature is the same for different values of (Formula presented.). The strain rate at extinction, (Formula presented.), is measured for values of (Formula presented.) up to 0.8. Computations are performed using detailed kinetic mechanisms and critical conditions of extinction and flame structures are predicted. The measurements and predictions show that, with increasingζst, the strain rate at extinction first decreases and then increases. The predictions agree with measurements for = ζst < 0,4, but significant deviations between measurements and predictions are observed at higher values of (Formula presented.). The scalar dissipation rate at extinction, χst,q is calculated using measured and predicted values of aq. With increasing (Formula presented.), the measured and predicted values of χst,q first increase and then decrease. It is noteworthy that changes in values of (Formula presented.) with (Formula presented.) for dimethyl ether flames are similar to those for methane flames, while the changes in values of (Formula presented.) with (Formula presented.) are remarkably different. Flame structures are predicted and they are found to be qualitatively similar to those for hydrocarbon fuels. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.