A sensitive, simple and rapid spectrophotometric method was developed for the determination of diphenhydramine in pharmaceutical preparation. The method was based on the charge-transfer complex of the drug, as n-electron donor, with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), as pi-acceptor. The formation of this complex was also confirmed by UV-vis, FTIR and (1)H NMR spectra techniques and thermal analysis. The proposed method was validated according to the ICH guidelines with respect to linearity, limit of detection, limit of quantification, accuracy, precision, recovery and robustness. The linearity range for concentrations of diphenhydramine was found to be 12.5-150 mu g/mL with acceptable correlation coefficients. The detection and quantification limits were found to be 2.09 and 6.27 mu g/mL, respectively. The proposed and references methods were applied to the determination of drug in syrup. This preparation were also analyzed with an reference method and statistical comparison by t- and F-tests revealed that there was no significant difference between the results of the two methods with respect to mean values and standard deviations at the 95% confidence level. (c) 2010 Published by Elsevier B.V.