The invention relates to a microcolumn reactor for carrying out reactions on solid phases and/or with biological cells. The aim of the invention is to provide a microcolumn reactor for processes of synthesis and separation on small sample volumina that replaces an alternating sequence of binding and elution processes on a phase that is stationary bound during a test series and makes it possible to exchange the stationary phase, for example once it has been loaded. To this end, the microcolumn reactor consists of at least one first and one second substrate wafer (1; 2) that are linked with one another across their surfaces. At least one of the substrate wafers (1; 2) is provided with an elongate channel (3) which comprises in a defined section (a) of its length two passage openings (41, 42) that are implemented in the opposite substrate wafer. Said openings (41, 42) are separated from the channel (3) by a partially permeable sieve-type membrane (5). The areas of acceptance (51) of said membrane have a diameter that is chosen in such a manner as to specifically prevent microbeads and/or cells introduced into the channel from entering the passage openings (41, 42). The channel (3) is provided with at least two further openings (61, 62) outside the section (a) that is comprised by the passage openings (41, 42). Said further openings permit the microbeads and/or cells that are provided above the section (a) to be introduced and/or displaced by applying a fluid pressure (p). Means are provided that temporarily close at least one of the passage openings (41, 42) and one of openings (61, 62).