An ultraviolet reactor (10, 100, 200, 300, 400, 500, 600) for irradiating a flow of fluid with UV radiation, the reactor comprising: a fluid conduit (20, 120, 220, 320, 420, 520) defined by a heat conducting conduit body (21, 121, 221, 321, 421, 521) comprising one or more heat conducting walls (24, 124, 224, 324, 424, 524, 624) for permitting a flow of fluid therethrough; a printed circuit board (40, 140, 240, 340, 440, 540, 640) comprising a heat conducting substrate (41, 141, 241) having a first surface (41A, 141A, 241A) comprising a circuit region (42, 142, 242); a solder mask coating (42A, 142A) in the circuit region (42, 142, 242) of the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241); and a UV light emitting diode (UV-LED) (30, 130, 230, 330, 430, 530, 630) operatively connected to the PCB in the circuit region (42, 142, 242), the UV-LED oriented for directing radiation into the fluid conduit (20, 120, 220, 320, 420, 520); wherein the heat conducting conduit body (21, 121, 221, 321, 421, 521) is in thermal contact with the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241) of the PCB; wherein a thermal contact region (44, 144, 244) of the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241) is a portion of the first surface (41A, 141A, 241A) that is in thermal contact with the heat conducting conduit body (21, 121, 221, 321, 421, 521); wherein the reactor is adapted to dissipate heat from the UV-LED via the heat conducting substrate, the thermal contact between the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241) and the heat conducting conduit body (21, 121, 221, 321, 421, 521), and from the one or more heat conducting walls of the heat conducting conduit body (21, 121, 221, 321, 421, 521) to the fluid in the fluid conduit (20, 120, 220, 320, 420, 520); and wherein the thermal contact region (44, 144, 244) is devoid of the solder mask coating (42A, 142A). a fluid conduit (20, 120, 220, 320, 420, 520) defined by a heat conducting conduit body (21, 121, 221, 321, 421, 521) comprising one or more heat conducting walls (24, 124, 224, 324, 424, 524, 624) for permitting a flow of fluid therethrough; a printed circuit board (40, 140, 240, 340, 440, 540, 640) comprising a heat conducting substrate (41, 141, 241) having a first surface (41A, 141A, 241A) comprising a circuit region (42, 142, 242); a solder mask coating (42A, 142A) in the circuit region (42, 142, 242) of the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241); and a UV light emitting diode (UV-LED) (30, 130, 230, 330, 430, 530, 630) operatively connected to the PCB in the circuit region (42, 142, 242), the UV-LED oriented for directing radiation into the fluid conduit (20, 120, 220, 320, 420, 520); wherein the heat conducting conduit body (21, 121, 221, 321, 421, 521) is in thermal contact with the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241) of the PCB; wherein a thermal contact region (44, 144, 244) of the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241) is a portion of the first surface (41A, 141A, 241A) that is in thermal contact with the heat conducting conduit body (21, 121, 221, 321, 421, 521); wherein the reactor is adapted to dissipate heat from the UV-LED via the heat conducting substrate, the thermal contact between the first surface (41A, 141A, 241A) of the heat conducting substrate (41, 141, 241) and the heat conducting conduit body (21, 121, 221, 321, 421, 521), and from the one or more heat conducting walls of the heat conducting conduit body (21, 121, 221, 321, 421, 521) to the fluid in the fluid conduit (20, 120, 220, 320, 420, 520); and wherein the thermal contact region (44, 144, 244) is devoid of the solder mask coating (42A, 142A).