There is considerable interest in the use of venturis to meter gas/solids flows. However, as there are two unknowns, the gas and solids flow rates, two pieces of information are required to calculate these values. It has been suggested that the pressure drop to the throat of the venturi and the recovery of pressure across the diffuser can be used for this purpose. A quasi-one-dimensional model for gas/solids flows in venturis has been developed. This model allows for the acceleration and deceleration of the gas and the solid particles as well as for changes in the thickness of the boundary layer. The last term is particularly important in the prediction of the pressure recovery in the diffuser. Predictions of the model have been validated against (previously published) experimental data. These show excellent agreement between the model and the experiments. In places the predictions are better than those from Computational Fluid Dynamics.