One of the major advantages of high field magnets is the increased signal-to-noise and spectral resolution for in vivo localized spectroscopy. In muscle studies, for example, the combination of proton and heteronuclear spectroscopy allows quantification of lipid content, fiber orientation, and cellular metabolic status. For clinical utilization it is important that spectra can be acquired in relatively short acquisition times for patient comfort and minimizing the effects of motion. Radiofrequency coils should ideally be fixed tuned and have relatively small effects with loading. Here, the design process and performance of a transformer-coupled double-tuned quadrature proton/phosphorus birdcage coil is described. The resulting coil is highly load-independent in terms of tuning, and despite the intrinsic reduced proton sensitivity compared to a single-tuned coil, clinically useful proton and phosphorus spectra can be acquired in a total time of less than 30 min. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 42A: 155–164, 2013.