When assessing particulate emissions, diesel engine exhausts are usually diluted to suit the design limitations of the measurement devices. Particle number concentrations (PNC) are known to be sensitive to dilution conditions and must be considered when evaluating results. Laboratories employ various experimental techniques to dilute exhaust samples before measurements. The majority of measurement systems use air as dilution a gas, some employ filtered exhaust gas in a closed loop, while others employ nitrogen, where prevention of oxidation reaction is required. In this work, the effect of using air and nitrogen as dilution gases on the PNCs from diesel engine exhausts has been investigated. Our approach explored the use of carbon dioxide (CO2) concentration ratios in diluted and raw exhaust samples, evaluated by non-dispersive infrared (NDIR) analysers to determine dilution conditions of the measured sample. The comparative effect of using nitrogen and air as dilution gases was then assessed. Comparison made with results previously obtained from using filtered exhaust as dilution gas in a closed loop system showed similar trends. PCN value was lower in air compared to nitrogen and by implication, the particle number concentration reduction factor (PCRF) and volatile removal efficiency (VRE) were higher with air.