In this report, the authors describe an extended corresponding states model for viscosity and thermal conductivity of mixtures implemented in Version 8 of the NIST computer program REFPROP. The model is a modification of a one-fluid, extended corresponding states (ECS) model for thermal conductivity and viscosity originally developed by Ely and Hanley The authors apply the model to selected mixtures representative of the fluids available in the NIST 23 database REFPROP (v8), such as refrigerants and natural gas constituent fluids, and present comparisons with experimental data. Comparisons are given for gas, liquid, and supercritical conditions.