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This paper describes the effects of recess dimensions on performance of recessed cathode dissolved oxygen (DO) microelectrode sensors. Microelectrodes with different recess dimensions were fabricated and characterized to establish the relationship between recess dimensions and sensor performance. The square-ring type cathode geometry inside the recess was formed with the recess length ranging from 35 to 246 mum and the length to hydraulic diameter ratio ranging from 3.16 to 12.26. Sensor performance was characterized by constructing calibration curves in 0.85% saline solution at three levels of dissolved oxygen (0%, 10%, and 21%). Response time, stirring effect (e.g., sensitivity to physical interference) and current sensitivity of each sensor were evaluated. By optimizing the DO sensor sensitivity, highly sensitive DO sensors may be fabricated without using ion selective membranes which complicate sensor fabrication and are prone to fouling.