The ethylene oxide monomer is nothing more than an epoxide ring. Two corners of the molecule consist of -CH2- linkages. The third corner is an oxygen, -O-. In the presence of a catalyst the monomer forms a chain having the repeat unit -CH2-CH2-O-. The PEO products found to be most effective in various papermaking applications are those having very high molecular mass, much in excess of one million grams per mole. Factors that make PEO wet-end chemistry puzzling include (a) the fact that it can be used as a retention aid even though it is nonionic, (b) the fact that its performance is highly dependent on its shear history, and (c) the fact that it needs the presence of lignin or certain phenolic "cofactor" additives to achieve its best effect as a flocculant and retention aid. Observations of PEO behavior suggest that the molecular chains are initially tangled with each other and that this tangling is somehow essential for effective flocculation. PEO is usually received as dry granules. These need to be dispersed with care, using a dilution ratio of at least 100 and avoidance of excessive shear.