Quantification of 31P NMR spectra is commonly performed using line-fitting techniques with prior knowledge. Currently available time- and frequency-domain analysis software includes AMARES (in jMRUI) and CFIT respectively. Another popular frequency-domain approach is LCModel, which has been successfully used to fit both 1H and 13C in vivo NMR spectra. To the best of our knowledge LCModel has not been used to fit 31P spectra. This study demonstrates the feasibility of using LCModel to quantify in vivo 31P MR spectra, provided that adequate prior knowledge and LCModel control parameters are used. Both single-voxel and MRSI data are presented, and similar results are obtained with LCModel and with AMARES. This provides a new method for automated, operator-independent analysis of 31P NMR spectra.