A study has been made at different relative humidities of the way in which electric charges on nylon threads decay. For relative humidities of less than about 35%, the charges remain localized and decay in magnitude at very nearly an exponential rate. For relative humidities greater than 35% the charges spread out along the thread and the decay of peak charge with time can be approximately represented by a t^-1/2 law rather than by an exponential one.If a diffusion equation of the type p = At^-1/2 exp (- x²/4 Dt) is applied to the latter results, values of the diffusion constant D are obtained which are independent, within the accuracy of the experiment, of (i) the sign of charge and (ii) whether it was a contact charge or a polarization charge. D varies exponentially with relative humidity.