This chapter summarizes some physical properties of ethylene oxide (EO). The structural parameters of EO as determined by microwave spectroscopy and electron diffraction. The principal chemical property of EO which is of concern in estimating biological risk is the same characteristic which renders the chemical of such enormous value as a chemical intermediate and which is probably also the quality that is responsible for its bactericidal activity. In biological systems the relative reactivity of EO with alcoholic OH groups as opposed to aliphatic amine groups of macromolecules is at least an order of magnitude lower. The biological effects of EO correlate with its chemical reaction pattern. The induction of point mutations and chromosomal damage by EO raises the question of nucleophilic selectivity. The suggestion is made that different molecular changes in DNA or damage to chromosomal proteins and/or mitotic apparatus are responsible for point mutations and chromosomal aberrations.