A process for preparing porous olefin polymers comprising bringing into contact in a polymerization reactor, at a temperature T.sup.1, one or more olefins of the formula (I) CH.sub.2.dbd.CHR.sup.1, R.sup.1 being hydrogen, a C.sub.1 C.sub.20-alkyl or a C.sub.6 C.sub.12-aryl group, with a catalyst obtained by reacting a solid catalyst component in the form of spheroidal particles comprising a compound of Ti or V not containing metal-.pi. bonds and a Mg halide, optionally containing one or more electron donor compounds, with an aluminum-alkyl (Al-alkyl) compound, then raising the temperature up to the polymerization temperature, said process being characterized in that: a) if the temperature T.sup.1 is lower than 40.degree. C., the temperature is raised up to the polymerization temperature is such a way that, for a time of at least one minute after the introduction temperature in such a way that, for a time of at least one minute after the introduction of the catalyst system into the reactor, the temperature of the reaction T.sup.2 fulfills the following condition of formula (II) wherein m is the time expressed in minutes employed for reaching the temperature T.sup.2; or b) if temperature T.sup.1 is 40.degree. C. or higher, the reactor is maintained for a time of at least one minute at a temperature of at least 40.degree. C.