The metrological criteria for the implementation of the single particle inductively coupled plasma mass spectrometry (SP-ICPMS) methodology applied to nanoparticle size characterization and quantification have been investigated. The SP-ICPMS basis involves a process of counting events corresponding to individual nanoparticles, which requires (i) isolation of the contribution of the nanoparticles from that of the background/dissolved analyte, and (ii) avoiding the occurrence of multiple-nanoparticle events. A criterion based on three times the standard deviation of the continuous background (3s) was selected as the threshold for discrimination of nanoparticle events from the background. Because the detectability of nanoparticles depends on both the size and number concentration, this 3s criterion was also selected for detection of nanoparticles at the size detection limit and concentrations over the number concentration detection limit. However, at very low number concentrations, a less restrictive criterion must be used. The selection of a critical nanoparticle number concentration, based on the sample introduction and data acquisition parameters, allows the minimization of the occurrence of multiple-nanoparticle events, as well as controlling of the precision associated with the counting of nanoparticle events. Under such conditions, the standard uncertainty associated with the determination of number concentrations was 5%. The uncertainty for the determination of nanoparticle diameters was also studied, varying from 3 to 10% for diameters in the range of 100–40 nm, respectively. Reliable average number concentrations and sizes were obtained, although the number size distributions showed a significant broadening contribution due to the SP-ICPMS measurement process. The feasibility of SP-ICPMS for the implementation of the European Commission definition of "nanomaterial" was studied by analyzing commercial silver nanoparticle suspensions.