The purpose of this study was to cross-validate measurements of skeletal muscle oxidative capacity made with near-infrared spectroscopy (NIRS) measurements to those made with phosphorus magnetic resonance spectroscopy (³¹P-MRS). Sixteen young (age = 22.5 ± 3.0 yr), healthy individuals were tested with both ³¹P-MRS and NIRS during a single testing session. The recovery rate of phosphocreatine was measured inside the bore of a 3-Tesla MRI scanner, after short-duration (∼10 s) plantar flexion exercise as an index of skeletal muscle oxidative capacity. Using NIRS, the recovery rate of muscle oxygen consumption was also measured using repeated, transient arterial occlusions outside the MRI scanner, after short-duration (∼10 s) plantar flexion exercise as another index of skeletal muscle oxidative capacity. The average recovery time constant was 31.5 ± 8.5 s for phosphocreatine and 31.5 ± 8.9 s for muscle oxygen consumption for all participants (P = 0.709). ³¹P-MRS time constants correlated well with NIRS time constants for both channel 1 (Pearson's r = 0.88, P < 0.0001) and channel 2 (Pearson's r = 0.95, P < 0.0001). Furthermore, both ³¹P-MRS and NIRS exhibit good repeatability between trials (coefficient of variation = 8.1, 6.9, and 7.9% for NIRS channel 1, NIRS channel 2, and ³¹P-MRS, respectively). The good agreement between NIRS and ³¹P-MRS indexes of skeletal muscle oxidative capacity suggest that NIRS is a valid method for assessing mitochondrial function, and that direct comparisons between NIRS and ³¹P-MRS measurements may be possible.