A process for preparing a pressure sensitive adhesive comprising:(A) an organic polymer having in its molecule at least one reactive silicon-containing group represented by the formula (I): ##STR1## wherein R.sup.2 is a monovalent hydrocarbon group having 1 to 20 carbon atoms or a triorganosiloxy group having the formula (II):(R').sub.3 SiO-- (II)in which R' is a monovalent hydrocarbon group having 1 to 20 carbon atoms and may be the same or different, X is hydroxyl group or a hydrolyzable group, and may be the same or different when 2 or more X groups are attached, a is 0 or an integer of 1 to 3, b is 0, 1 or 2, and m is 0 or an integer of 1 to 18, and(B) 0.1 to 20 parts by weight of, per 100 parts by weight of the organic polymer, at least one curing catalyst selected from the group consisting of an organic aluminium and an organic zirconium. The composition has excellent heat resistance and releasability from substrates, e.g. a release paper coated with a silicone release agent.