The site of hydration for 1,6- and 1,7-naphthyridine has been determined by specific line broadening in the 1H n.m.r. spectra on the addition of water. The site of hydration appears to be the same as the site of protonation (ß-nitrogen atom). The site of hydration in quinazoline has been shown to be the α-nitrogen atom. This strongly indicates that the site of protonation in this compound will also be at that nitrogen.