Spherical (23) or cylindrical (27, 36) electrodeless ultraviolet lamps are used to remediate fluid, directly or by excitation of ultraviolet-activated photocatalyst surfaces, which may be on the lamps themselves, or on structures which are permeable by the fluid. The lamps may be excited in cavities (18, 19, 43) by microwave energy from magnetron (22), or by radio frequency power (39) inductively coupled (40) to the lamps. The lamps (44) may have start-up electrodes (47).