Appts. for producing ultra-fine crystalline products includes (i) supply vessels (1,2) equipped with temp. measuring instruments (T1,T2) and connected by lines (a,b) to coupled dosing pumps (3); (ii) lines (c,d) including volumetric flowmeters (V1,V2) and connecting the pumps (3) to a static mixer (4) which is connected by a line (e) to a centrifugal separator (5) having a tachometer (IV); (iii) valves (4.1-4.n) which control the static mixer (4) and which are connected to and controlled by a control module (7); (iv) a line (f) including a product quantity measuring instrument (M) and connecting the centrifugal separator (5) to an instantaneous particle size sensor (6) coupled to the module (7), the line (f) being the crystallisate discharge line; and (v) a line (g) with a volumetric flowmeter (V3), used to discharge effluent from the centrifugal separator. USE/ADVANTAGE - The appts. is useful for prodn. of pigments for colours and paints; ultra-fine chemicals for the electronics, optics and photographic industries and biotechnology; drugs such as antibiotics and hormone prepns.; and substances for special uses. It provides more precise particle size control than conventional pptn. crystallisation appts..