The mineralogy of the hydrothermally altered basalts of the Kailua Volcanic Series from Keolu Hills, Oahu, Hawaii, was examined by X-ray diffraction analysis. Vertical and horizontal clay-mineral alteration zones were found. In the vertical zones, mixed-layer chlorite-vermiculite occurs more commonly than chlorite, whereas in the horizontal zones, mixed-layer chlorite-montmorillonite and chlorite are both more common. The occurrence of the two clay minerals kaolinite and mixed-layer chlorite-montmorillonite in the soil is not related to the parent rock but to differences in drainage. The zeolites are calcium zeolites which commonly coexist with quartz; laumontite is the dominant zeolite in the study area. The widespread distribution of laumontite in the Kailua Volcanic Series suggests that low temperatures (approximately 300 degrees C) prevailed during the formation of secondary minerals and indicates that as much as 3,500 feet of weathering has occurred since caldera formation.