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Aqueous urea solutions are being considered for use on light-duty diesel vehicles to reduce engine-out NOx to 2007 Tier II Bin 5 levels (0.07 g/mi NOx). A 32.5 wt% urea solution freezes at approximately 12°F (-11°C). This is a challenge for use in colder climates. Propylene glycol is a safe and effective freeze point depressant, providing protection down to 0°F (-18°C). A solution of aqueous urea and propylene glycol was used during a series of transient dynamometer experiments performed with a 1.2 L diesel engine. A detrimental effect on NOx conversion was observed, and tailpipe emissions of HC dramatically increased. Results were confirmed in a laboratory flow reactor, along with effects of other proposed additives such as methanol and ethanol.