Byne Hill, two miles south of Girvan, Ayrshire, is composed of Lower Ordovician igneous rocks which form partof the Girvan-Ballantrae igneous series. Byne Hill consists of serpentinized harzburgite, gabbro, diorite and trondhjemite which probably form a N.E.–S.W. elongated dome with trondhjemite occupying the core. There are transitional passages from gabbro into dioritic gabbro, diorite, quartz diorite and trondhjemite with no sharp contacts, and gabbroidal minerals and textures persist into the marginal hornblendic phases of the trondhjemite. The mineralogical changes are mainly progressive amphibolization and albitization of the gabbro accompanied by increases in SiO2 and Na2O. Geochemical culminations of FeO, MnO and P2O5 occur in the intermediate dioritic gabbro and diorites. Comparing the intermediate rocks with similar rock series from other areas suggests that mineralogical and chemical criteria do not always distinguish intermediate rocks of metasomatic origin from those produced by fractional crystallization. On Byne Hill the textural evidence suggests that the intermediate rocks are hybrids produced by reaction between crystalline gabbro and silicic sodium-rich solutions related to the trondhjemite, although there is evidence that the trondhjemite is itself metasomatic in origin. Serpentinization and rodingitization of the Byne Hill gabbro at contacts with serpentinite are described, and it is concluded that the gabbro was emplaced into the ultrabasic rocks prior to their serpentinization.