Abstract A pre-formulation study was directed to optimize the in vitro floating ability of an air compartment multiple-unit system. Each unit was formed by a coated bead composed of a calcium alginate core separated by an air compartment from a calcium alginate or calcium alginate/polyvinylalcohol (PVA) membrane. The floating ability depended on the presence of the air compartment and on membrane porosity. The porous structure generated by the leaching of PVA, employed as a water-soluble additive in the coating composition, increased the membrane permeability preventing air compartment shrinkage. In this way, units were produced which were able to float immediately upon contact with artificial gastric juice and for a long period of time. The floating ability increased with the increase in PVA concentration and molecular weight and it was found to be excellent when using PVA 100 000 at a concentration of at least 5%.