The objective of this work is to develop a milling model for a continuous self-classifying spiral air jet mill. Its foundation is a population balance model with selection and breakage distribution functions that have been related to a minimal number of mill-dependent and powder-dependent parameters. Initially, experimentation is required to determine the mill-dependent parameters for a specific mill, by milling a base powder at multiple operating conditions. Powder-dependent parameters can be determined from either mill experiments or from material characterization measurements that require small amounts of powder (presented in Part 2). Ultimately, the milling model presented successfully predicts the product particle size using as inputs the feed particle-size distribution and mill operating conditions. Three crystalline powders, sodium bicarbonate, lactose monohydrate, and sucrose, have been used to test the proposed milling model. (c) 2014 American Institute of Chemical Engineers AIChE J 60: 4086-4095, 2014