An electrotransport delivery device utilizing reservoir buffering at a select pH ranges in order to reduce skin irritation and skin resistance is provided. Cathodic reservoirs are buffered to a pH of less than about 4, preferably to a pH in the range of about 2 to 4, while anodic reservoirs are buffered to a pH above about 4, preferably to a pH in the range of about 4 to 10. Another electrotransport delivery device utilizes a potassium sensor to monitor potassium efflux from the skin. Potassium efflux above a certain predetermined level has been found to be a precursor to skin irritation/erythema. Operation of the device is modified (eg, terminated) when the predetermined potassium efflux level is sensed.