The purpose of this study was to investigate the in ̄uence of relative humidity (RH) on the mechanical and dissolution properties of theophylline pellets coated with Eudragitw RS 30 D/RL 30 D containing methylparaben (MP) as a non-traditional plasticizer. The coated beads were stored at 23VC and at different relative humidities (0, 29, 51, 75 and 84% RH). The effect of storage conditions on the rate of drug release from coated beads was determined in pH 7.4 phosphate buffer solution. The mechanical properties, including tensile strength and Young's modulus, of individual beads were determined by a diametral compression method with a Chatillonw tension/compression appa- ratus. The morphology of the intact and fractured beads was investigated using scanning electron microscopy (SEM). The moisture content of the polymeric ®lms was determined using a Karl Fischer coulometric moisture analyzer. The results from the mechanical studies demon- strated that an increase in the relative humidity resulted in a decrease in the tensile strength and Young's modulus of the coated beads. SEM photographs showed that coated beads stored at 0% RH exhibited brittle fracture failure. The coated beads stored at 84% RH showed ductile behavior, which was attributed to the hydroplasticization effect on the acrylic polymer due to the uptake of moisture. The moisture content in the ®lms was also shown to in ̄uence the rate of drug release from Eudragitw RS 30 D/RL 30 D coated beads containing MP as the plasticizer. The change in release pro®les could be minimized when the relative humidity was reduced to zero. The dissolution rate of theophylline from the coated beads decreased when stored at high relative humidities. This trend was reversed when the coated beads that were stored at 84% RH for 5 weeks, were then equilibrated at 0% RH.