A photocatalytic reactor for carrying out a photocatalytic reaction on a liquid which is to be treated, the reactor comprising a reaction chamber which comprises: (i) a foraminated member which supports a plurality of mobile photocatalyst particles, the size and density of which is such that they tend to rest on the foraminated member in use; and (ii) an aeration device which causes gas bubbles to rise from the foraminated member and agitate the mobile photocatalyst particles. The reactor may be a flow-through reactor. The photocatalytic reactor can be applied to the remediation of waste water using titanium oxide.