Volume III/28 is a supplement to volume III/16a+b Ferroelectrics and Related Substances. It also consists of two parts a and b. The present subvolume III/28a contains data on ferroelectric oxides, supplementing III/16a which appeared in 1981. Subvolume III/28b, due in 1990, will cover non-oxides, supplementing III/16b (published in 1982). Reliable data on both pure compounds and solid solutions, published mostly between 1978 and 1986 and some data from the literature up to early 1988 are critically evaluated and included. The dielectric and ferroelectric behaviour, as well as all other properties relevant to the characterization of these substances are presented in tables and figures. About 22,000 references have been surveyed with the aid of a computer. All values are given in SI units. Rapid localization of the required data is facilitated by an alphabetical index of substances and a two-dimensional survey of substances and properties dealt with in both subvolumes III/28a and III/16a.