The small size range (1-100 nm) of many radiocolloids [used in diagnosis and therapy] and their polydispersity limit on the choice of the technique used were studied. To compare several techniques directly, the particle size of 99mTc sulfide colloid was studied by using Nuclepore filtration, ultracentrifugation and EM. The last of these was adopted as the method of choice. Using this technique, the particle size and shape of colloids below 100 nm were accurately determined. 99mTc-antimony sulfide colloid and 113mIn hydroxide were then examined by EM and the chemical nature of the particles was determined by X-ray fluorescence analysis. Results resolved the size discrepancies reported and demonstrated the importance of identifying the chemical nature of the particles under examination.