The TSI Fast Mobility Particle Sizer (FMPS), Engine Exhaust Particle Sizer (EEPS), and Scanning Mobility Particle Sizer (SMPS) provide size distributions for 6–560 nm particles. The aim of this study was to perform comprehensive equivalence testing of these three particle sizing instruments with particles of contrasting chemical and physical characteristics (urban ambient, diesel exhaust, and laboratory-generated particulate). It was observed that the EEPS and FMPS measurements agreed to within 15% thus concluding that data from these instruments may be considered equivalent. Parallel measurements with the SMPS showed that when measuring diesel exhaust particulate during ISO8178 Mode 9 operation there is significant overestimation of particle concentrations by both the EEPS and the FMPS in the 20–120 nm size range (25–38% overestimation). This overestimation also occurred for near-road measurement of heavy emitter vehicle plumes in ambient samples (up to 75% overestimation). Laboratory-generated soot agglomerate particles, whose shape was verified by transmission electron microscopy, were also tested. The agglomerate nature of diesel soot particulate was the dominant cause of the overestimation; parallel measurements with an FMPS and an Ultrafine Condensation Particle Counter of the laboratory-generated soot particulate showed overestimation by up to a factor of three.