Colloidal particle size is an important characteristic to consider when choosing a radiopharmaceutical for diagnosis and therapeutic purposes in nuclear medicine. Photon correlation spectroscopy (PCS) and transmission electron microscopy (TEM) were used to determine the particle-size distribution of 90Y- and 99mTc-labelled antimony trisulfide (Sb2S3) and tin colloids (Sn-colloid). 90Y-Sb2S3 and 99mTc-Sb2S3 were found to have a diameter of 28.92 ± 0.14 and 35.61 ± 0.11 nm, respectively, by PCS. By TEM, 90Y-Sb2S3 particles were measured to be 14.33 ± 0.09 nm. 90Y-labelled Sn colloid were found to exist with a dv(max1) of 805 nm and a dv(max2) of 2590 nm, by PCS, whereas 99mTc-Sn colloid was shown to have more than 80% of radioactive particles of approximately 910 nm by PCS. For 90Y-labelled Sb2S3 and Sn colloid, a comparison of TEM and PCS indicates that these techniques found significantly different mean diameters. TEM has an excellent resolution necessary for radiocolloid particle-sizing analysis, and it is a desirable size-measuring technique because it is more reliable than PCS.