Abstract: Sustained-release coated pellets with different sizes (6, 2.5 and 1 mm in diameter) were investigated using terahertz pulsed imaging (TPI). Three batches of metoprolol succinate layered sugar starter cores coated with a 75:25 (w/w) polymer blend of Kollicoat SR and Kollicoat IR (approximate coating thickness of 60 μm,according to the weight gain) were analysed to evaluate the effect of size on coating thickness and morphology (depicted by the terahertz electric field peak strength, TEFPS) Ten pellets from each batch were mapped individually using TPI. From the terahertz waveform the interface between coating and drug layer, and between drug layer and core were determined. The TPI measurements were carried out on pellet surface areas of approximately 33, 2.2 and 0.4 mm² for pellets with 6, 2.5 and 1 mm diameters, respectively. Results indicated a large variation in the average coating thickness between all pellet sizes. Smaller pellets (2.5/1 mm in diameter) showed a higher average coating thickness (81 and 70 μm, respectively) compared to 6 mm pellets (50 μm), suggesting a better coating efficiency for smaller pellets. This was also confirmed by scanning electron microcopy (SEM). Since no difference in the surface morphology could be observed using SEM, differences in the average TEFPS values between 6 mm pellets (16.2%) and 2.5/1 mm pellets (2.2 and 2.6%, respectively) are related to signal reflection loss due to the increase in curvature of smaller pellets. Although the largest pellets showed the thinnest average coating, the fastest drug release was obtained from the smallest pellets due to the larger surface area exposed to the dissolution media. Pellets of 2.5 mm in diameter showed a faster initial drug release with slower release kinetics at the end of dissolution testing compared to large pellets. TPI proved highly suitable to evaluate film coating characteristics as well as detect drug layer/core interface of different sized sustained-release coated pellets.