A filter apparatus (10) for cleaning functional fluids, especially selected from phosphate esters, phosphate ester-based oils, mineral oils and phenols, includes a filter housing (12) into which a filter bed (14) has been introduced. The filter bed has a mixture of anionic and cationic ion exchange resins. At least one filter element (16), designed as a flow guide device, enables the diversion of the fluid from a radial to an axial flow direction (18) or vice versa.