The construction and calibration is described of a new type of pH glass electrode with a measuring bulb of 1.0 mm in diameter. The lower one-fourth zone of the bulb, a 15 to 70µm thick membrane of pH glass, contributed at least 70% to the final reading, hence allowing a spatial resolution down to 100 µm. The 90% response time depended on temperature and varied from 20 s at 0 °C to 5 s at 36 °C in buffer solutions and from 5 to 15 s in sediment at 18 °C. A maximal deviation of 4% resulted from the use of standard buffers for calibration instead of saline buffers. Over the temperature and pH range tested (0 to 36 °C and pH 7 to 10) the mini-electrode showed a linear response. Responses varied from 91 to 98% of those calculated from the Nernstian equation. In melting ice 75% of the Nernstian response was still found. The sturdy bulb of the mini-electrode allows its use in muddy as well as sandy sediments.