Peri-therapeutic (124)I-PET/CT is of interest as guidance for radioiodine therapy. Unfortunately, image quality is complicated by dead time effects and increased random coincidence rates from high (131)I-activities. A series of phantom experiments with clinically relevant (124)I/(131)I-activities were performed on a clinical PET/CT-system. Noise equivalent count rate (NECR) curves and quantitation accuracy were determined from repeated scans performed over several weeks on a decaying NEMA NU-2 1994 cylinder phantom initially filled with 25 MBq (124)I and 1250 MBq (131)I. Six spherical inserts with diameters 10-37 mm were filled with (124)I (0.45 MBq ml(-1)) and (131)I (22 MBq ml(-1)) and placed inside the background of the NEMA/IEC torso phantom. Contrast recovery, background variability and the accuracy of scatter and attenuation corrections were assessed at sphere-to-background activity ratios of 20, 10 and 5. Results were compared to pure (124)I-acquisitions. The quality of (124)I-PET images in the presence of high (131)I-activities was good and image quantification unaffected except at very high count rates. Quantitation accuracy and contrast recovery were uninfluenced at (131)I-activities below 1000 MBq, whereas image noise was slightly increased. The NECR peaked at 550 MBq of (131)I, where it was 2.8 times lower than without (131)I in the phantom. Quantitative peri-therapeutic (124)I-PET is feasible.