Radioactive particle tracking is a nonintrusive technique that has been successfully used to study the flow dynamics in a wide range of reactors and blenders. However, it is still limited to the tracking of only one tracer at a time. A multiple radioactive particle tracking (MRPT) technique that can determine the trajectory of two free or restricted (attached to the same particle) moving tracers in a system is introduced. The accuracy (<5 mm) and precision (<5 mm) of the proposed technique is evaluated by tracking two stationary tracers and two moving tracers. The results confirm the reliability and validity of the MRPT technique when the two tracers have the same isotope and the distance between them is not too small (>2 cm). The tracking of two sticking tracers at the two ends of a cylindrical particle in a rotating drum is also considered to illustrate the potential of this characterization method. (c) 2014 American Institute of Chemical Engineers AIChE J, 61: 384-394, 2015