A mixing reactor (1), the reactor comprising a body (2) having a first inlet (3), a second inlet (5) and an outlet (4), in which there is an inner passage (6) through the body (2) from the first inlet (3) at a first end (12) of the body (2) to the outlet (4) at a second end of the body (2) along a length of the body (2), the inner passage (6) having a side wall (17) along the length, and an outer passage (7) closer to a surface (14) of the body (2) than the inner passage (6), the outer passage (7) running from the second inlet (5) at the second end, travelling through the body (2) along the length and meeting the inner passage (6) at a junction (11) at the first end, the outer passage (7) joining the inner passage (6) through the side wall (17) at the junction (11). Such a mixing reactor can be used for producing particles such as nanoparticles or Metal-Organic frameworks. Furthermore, we disclose a cascade of such reactors and a method of using such reactors to mix fluids, typically but non-exclusively so as to produce such particles.