Fourteen samples of nickel-containing serpentine minerals, all of which resemble lizardite, have been studied by X-ray powder diffraction, thermogravimetric and chemical analysis, and electron microscopy. Those containing more than 1.5 Ni ions per formula unit R3Si2O5 (OH)4 are called nepouites, and those with fewer Ni ions are called nickel lizardites. The samples vary considerably in structural order and the nepouites generally are less well-ordered than the nickel lizardites. Among the latter, two show clear evidence for a 2-layer orthogonal structure, and the diffracted intensities correspond generally with the 2Or polymorph described by Bailey. Thermogravimetric analysis of these minerals shows that hydroxyl ions are retained after the first stage of reaction when the 7.2 Å basal spacing is lost and a long spacing, LS-phase appears with a spacing in the range 14-11 Å. It is suggested that this phase is similar to a chlorite after the first stage of dehydroxylation. All the samples studied show, to varying extents, the formation of an LS-phase. Nepouites also show a second transitional phase, which is face-centered cubic, and appears to be a finely divided form of nickel oxide. The high-temperature phases are olivine and enstatite from low-nickel materials, and olivine and cristobalite from high-nickel materials.