Simultaneous measurements of flare-enhanced electron density in the E layer and of flare-enhanced solar X-ray flux were used to evaluate the ionospheric relaxation time constant at E-layer heights. The method is an extension of Appleton's ionospheric “filter” idea to the case in which radiation flux and electron density fluctuations are simultaneously measured. Electron density measurements were made using the Thomson scatter technique at the National Astronomy and Ionosphere Center, Arecibo, Puerto Rico. X-ray flux data in the 2- to 12-Å band were obtained from instruments aboard the Explorer 33 satellite. Measured values of time constant were converted to an effective recombination coefficient using the familiar Appleton formula and known values of the ambient electron density. Values for the effective recombination coefficient of the order 10−7 cm3/sec were obtained at E-layer heights.