Iodine-123 (159 keV, T1/2 = 13.3 hr) has been proposed for renal investigations, as opposed to I-131 (364 keV, T1/2 = 8.06 days), because of its more practical photon energy and lower radiation dose to the patient. The cyclotron production method 124Te (p,2n) 123I for I-123 results in contamination with I-124 (T1/2 = 4.5 days). The latter emits high-energy photons whose relative abundance increases with time after end of bombardment (EOB). This paper is an evaluation of the effects of photon penetration, scatter, and attenuation on the phantom calibration measurements required for determining relative renal uptake using I-123. Measurements using I-131 were performed for comparison. Parameters investigated included: (a) the relationship between the integrated count and ROI size, (b) the magnitude of the "cross-talk" in counts between the kidneys, and (c) the attenuation corrections for source (kidney) depth. Phantom results obtained for I-123 suggest that this radionuclide will allow a better measurement of the activity in individual kidneys. Collimator penetration effects were greater for I-131 than for I-123. With I-123 for example, a higher fraction of the counts due to the activity in the kidney phantom were observed within the ROI enclosing its image. However, the attenuation corrections for source depth for I-123 were dependent on both the size of the ROI and time after EOB.