Aliphatic amines react with phosphoimidazolide-activated derivatives of guanosine and cytidine (ImpN) by replacing the imidazole group. The kinetics of reaction of guanosine 5'-phospho-2-methylimidazolide (2-MeImpG) with glycine ethyl ester, glycinamide, 2-methoxyethylamine, n-butylamine, morpholine, dimethylamine (Me2NH), ethylmethylamine (EtNHMe), diethylamine (Et2NH), pyrrolidine, and piperidine were determined in water at 37 C. With primary amines, a plot of the logarithm of the rate constantfor attack by the amine on the protonated substrate, log ŋsiA versus the p of the amineexhibits a good linear correlation with a Brpnsted slope,/3nuc= 0.48. Most of the secondary aminestested react with slightly higher reactivity than primary amines of similar pKa. Interestingly, some secondary amines show substantially lower reactivity than might be expected: EtNHMe reacts about eight times, and Et2NH at least 100 times, more slowly than Me2NH although all threeamines are of similar basicity. For comparison, the kinetics of reaction of guanosine S'-phosphoimidazolide (ImpG) and cytidine S'-phosphoimidazolide (ImpC) were determined with Me2NH, EtNHMe, and Et2NH, and similar results were obtained. These results establish that the increased steric hindrance observed with the successive addition of ethyl groups are not due to any special stericrequirements imposed by the guanosine or the methyl on the 2-methylimidazole leaving group of 2-MeImpG. It is concluded that addition of ethyl and, perhaps, groups largerthan ethyl dramatically increases the kinetic barrier for addition of aliphatic secondary amines to the PN bond of ImpN. This study supports the observation (see ref 4b) that the primary amino groups on the natural polyamines are at least 2 orders of magnitude more reactive than the secondary amino groups in the reaction with ImpN.