This paper discusses inferential measurement models for semiautogenous grinding (SAG) mills. Inferential measurements of SAG mill discharge and feed streams and mill rock and ball charge levels are obtained utilising process measurements and recognised process simulation models. Inferential models of recirculating load and cyclone underflow split are also presented. Results for the mill inventories and process streams are validated against reference simulation model data. Uncertainty analyses are conducted to assess the influence of the various model parameters. Mill weight based estimates for mill inventory are shown to be the least uncertain. The results suggest that regular calibration of oversize crusher and primary cyclone feed instrumentation, regular measurement of the SAG mill discharge screen aperture, oversize crusher gap setting and process water specific gravity and careful fitting of the SAG mill discharge grate model parameters will minimise uncertainty in the inferential models. (C) 2002 Elsevier Science Ltd. All rights reserved.