[en] 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. (paper)