The elimination of water condensation is a familiar challenge to exhaust sampling system design and operation. The Constant Volume Sampler (CVS), commonly used to sample automotive exhaust, uses ambient air to dilute raw exhaust in order to keep water vapor, produced by the combustion process, in suspension. Without sufficient dilution, the water vapor will condense on the surfaces of the CVS and sample bags, absorbing water soluble compounds. These compounds are left unanalyzed resulting in nonrepresentative emissions and fuel economy measurements. Furthermore, condensation in the bag sampling system (pump, filter, lines, etc.) will increase all emissions measurements. Overdilution of the raw exhaust, on the other hand, produces low-level diluted concentrations which are more difficult to accurately measure. Therefore, optimal dilution is emerging as a critical feature of emissions testing as regulatory mandates require vehicles to significantly reduce emissions while operating under more rigorous conditions.This paper develops a basis for predicting a proper amount of dilution from well-known properties of the vehicle and test cycle used. The predictions are verified from measurements of the dew points of collected samples. Recommendations for proper setting and design of sampling systems are made.