The effect of increasing primary amine hydrocarbon chain length on the SCR of NOx from diesel engine exhaust was investigated and compared to ammonia. Methylamine (CH3NH2), ethylamine (CH3CH2NH2), propylamine (CH3CH2CH2NH2) and butylamine (CH3CH2CH2CH2NH2) were tested using a 12 cell mini core NH3 - SCR catalyst cut from a 400 cpsi block. There is a steady decrease in NOx conversion as the length of the hydrocarbon chain increases (from 50% for methylamine to 26% for butylamine). For the same number of carbons in the amine, primary amines are more active reductants than methyl substituted secondary or tertiary amines. For example, ethylamine (NOx conversion of 45%) is more active than dimethylamine (NOx conversion of 34%). Since the amines are reactive in the gas phase in the temperature range of diesel engine exhaust, gas phase conversions were estimated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. There was no smooth transition in gas phase NO and NOx conversions with increasing hydrocarbon chain length. The results suggest a different mechanism for gas phase reactions depending on the nature of the amine. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.