Rationale and Objectives. The influence of partial tumor sampling in a volume of interest (VOI) on the ratios of phosphorus metabolites was examined by localized phosphorus magnetic resonance spectroscopy (P-31-MRS). Experiments were performed to investigate the characteristics of the surface coil used and precession of spatial localization. Methods. A total of 24 patients with liver metastases and 20 volunteers were studied by P-31-MRS. Patients were divided in two groups: VOI < 50% (n = 8) and VOI > 50% (n = 16) occupied by tumor. For evaluation of the surface coil and localization method (image selected in vivo spectroscopy), phantom studies were performed. Results. Superficial focal liver tumors were detectable with a surface coil at a distance within the coil radius. The image selected in vivo spectroscopy permitted the study of phosphorus metabolism in a defined VOI. phosphomonoester/ β-adenosine triphosphate and phosphodiester/β-adenosine triphosphate were elevated significantly in spectra of both patient groups. Conclusions. Detection of small tumor volumes within a VOI filled by less than 50% of the tumor is possible, with results statistically different from that in normal volunteers.