Partially coated perforated systems (PCPS) based on low-viscosity hydroxypropyl methylcellulose (HPMC) as the polymeric material were formerly designed, prepared and evaluated in terms of in vitro behaviour. These systems proved to afford the pursued linear release with model drugs (metoprolol tartrate and benfluorex) of different solubility. To the aim of exploring the mechanisms concurring in the definition of zero-order kinetics, studies of drug release, polymer dissolution and medium penetration were performed on PCPS and constant release area systems (CRAS). According to the obtained results, PCPS release kinetics has to be mainly attributed to the progressive outward erosion of the core and to the related variation of the release area. The special geometry of the system, in fact, involves a gradual increase in the release surface, which allows the diffusional path lengthening to be offset. By properly selecting the shape and dimensions of the device as well as the physico-chemical characteristics of the hydrophilic polymer, the advantage of a zero-order release kinetics with programmable rate can be achieved.