The design of a novel photoelectrochemical sensor, the micro-optical ring electrode (MORE), is described, Based on a thin-ring microelectrode and using a fibre-optic light guide as the insulating material interior to the ring, the MORE is capable of delivering light directly to the region of electrochemical measurement and can therefore be used to conduct microelectrochemical studies of systems with complex photochemistry. A novel fabrication procedure is described, involving the coating of commercially available fibre optics (radius 1.25 x 10(-4) m) with a 600 nm layer of gold, so allowing exploitation of the electroanalytical advantages peculiar to thin-ring microelectrodes. The dark electrochemistry of the thin-ring microelectrode is characterized by use of cyclic voltammetry and chronoamperometry and found to agree with previously published theoretical results, Preliminary exploration of the photoelectrochemical response of the MORE is reported, achieved via the interrogation of the photoelectrochemically active phenothiazine dye methylene blue (MB(+)), Photocurrent signals obtained during cyclic voltammetric and chronoamperometric: studies of MB(+), conducted with the MORE under illuminated conditions and in the absence of any deliberately added reducing agent, are attributed to the formation and subsequent detection of (3)MB(+) within the diffusion layer of the microring electrode, The data demonstrate that the use of the MORE for the direct electrochemical detection of photogenerated species with lifetimes of <9 x 10(-5) s is possible, The electrochemistry of (3)MB(+) over the applied potential range from -0.4 to +1.0 V versus SCE is elucidated and discussed in the context of the behaviour of photoexcited MB(+) in the presence of the deliberately added reducing agent Fe2+.