Silica sol-gels have been chosen in the previous two chapters to illustrate the range of possible mechanistic and chemical studies in sol-gel science. However, the review of the historical development of the subject in Chapter 1 demonstrates that many of the earliest uses of the sol-gel process involved metal oxide gels rather than silica gels. This chapter will examine the chemistry of these materials, and their applica­ tions will be discussed in Chapter 7.