An experimental investigation has been undertaken to determine the effects of moisture sorption propensity on the triboelectrification of pharmaceutical excipients in contact with materials used for the fabrication of pharmaceutical processing equipment and drug delivery devices.Moisture sorption and desorption isotherms were determined for a-lactose monohydrate, dextrose monohydrate, spray-dried sorbitol and sodium starch glycolate. The isotherms show that alpha-lactose monohydrate and dextrose monohydrate are non-hygroscopic over a wide range of relative humidity (rh) from 0% to 80% and have negligible hysteresis on desorption. Sodium starch glycolate was the most hygroscopic, spray-dried sorbitol was of intermediate hygroscospicity and both isotherms showed hysteresis on desorption. The sorption data was analysed to estimate the distribution of monolayer adsorbed, multilayer adsorbed and absorbed moisture. The effect of storage relative humidity on the triboelectrification of the powders was investigated using steel, polypropylene and polyvinylchloride (PVC) as contact surfaces. The results showed that moisture sorption and distribution data could be useful in predicting the electrostatic charging of pharmaceutical powders. There was negligible effect of rh on the charging of the excipients with low hygroscopicity, whereas the charging of the sodium starch glycolate was inversely related to the storage rh. The relationship between moisture sorption/distribution and electrostatic charging for spray-dried sorbitol was more complex and attributed to the greater distribution of sorbed water in the pore structure of the sorbitol. The non-hygroscopic excipients charged negatively with steel and positively with PVC and polypropylene, whereas sodium starch glycolate charged negatively with all contact surfaces. (C) 2003 Published by Elsevier B.V.