Prostanoic acid esters of the formulaPG-- CH.sub.2 -- X-- Ywherein PG is the prostanoyloxy radical of a prostaglandin, X is a carbon-carbon single bond, carbonyl or carbonyloxy, and Y is substituted phenyl are easily crystallized compounds at least as active as the unesterified prostanoic acid and useful for the purification of the parent prostaglandin are prepared by reacting, in the presence of an agent which splits off hydrogen halide, the unesterified prostaglandin with a halide of the formula Hal--CH.sub.2 --X--Y, wherein Hal is a halogen atom and X and Y have the values given above.