A microreactor for performing chemical reactions, includes a body (2, 3), a first chamber (5) and a second chamber (7), both formed in the body (2,3). Interconnections (8) are provided for fluidly coupling the first chamber (5) and the second chamber (7) through the body (2, 3). The microreactor also includes a venting passage (10; 110; 210) formed in the body (2,3) and having a venting outlet (10b) in the second chamber (7) and at least one venting inlet (10a) in the first chamber (5), at a location where formation of bubbles is expected upon introduction of a liquid in the first chamber (5).