A process for preparing a 6-methylene-.DELTA..sup.4 -3-keto steroid of the formula ##STR1## wherein R is hydrogen, alkoxy of up to 6 carbon atoms or acyloxy of up to 6 carbon atoms wherein the acyl group is that of a carboxylic acid, andR' is the CD-ring system of a steroid of the androstane or pregnane series,comprising reacting the corresponding .DELTA..sup.4 -3-keto steroid of the formula ##STR2## with a formaldehyde derivative of the formulaX(CH.sub.2 O).sub.n Ywhereinn is 1, 3 or an integer on the order of 100-1000, andX is C.sub.1-5 alkoxy and Y is C.sub.1-5 alkyl when n is 1,X and Y represent a single bond between the terminal C atom and the terminal O atom when n is 3, andX is hydroxy and Y is hydrogen when n is an integer on the order of 100-1000,in an inert solvent in the presence of a condensation agent which is a strong acida strongly acidic cation exchanger or a phosphoric acid derivative.