This paper discusses the combined state and parameter estimation of SAG mill inventories and model parameters. Recognised simulation models are utilised for the rock and water charge state equations. New models of the ball charge and mill shell lining states are presented. The 36 state system is detectable although not completely observable. Five ore grindability and mill discharge grate parameters augment the state system. One mill weight and two discharge measurement models are presented and utilised in two state estimation formulations. Results indicate that a size by size SAG mill discharge measurement model provides superior state estimates and improved discharge grate parameter estimation compared to a bulk SAG mill discharge measurement model. (C) 2002 Elsevier Science Ltd. All rights reserved.