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D-region electron densities have been measured with a height resolution of less than 1 km using the incoherent scatter technique with coded pulses. Using a theoretical production model and the measured densities, recombination coefficients were computed for the altitude range from 80 to 95 km. The effective recombination coefficient decreases across the D-region ledge from a value of about 2 × 10−5 cm3 sec−1 below the ledge to a value of the order of 1 to 5 × 10−7 above. The change is presumably due to a transition from water cluster ions to the molecular ions NO+ and O2+.