A non-invasive determination of Na+ and K+ concentrations in skeletal muscle tissue is desirable to gain insights into pathological processes connected to various diseases. In this work, the feasibility of combined quantitative 23Na/39K MRI at 7 T using a double-tuned 23Na/39K birdcage calf coil was examined. In phantom measurements, a 23Na/39K SNR ratio of 46.8 was found. Moreover, Na+ and K+ concentrations close to the real concentrations were determined. In skeletal muscle tissue, fast transverse relaxation of 39K leads to underestimation of K+ concentrations if no relaxation correction is applied.