In this study, we investigate the impact of polymer concentrations on the characteristics of duplex microcapsules prepared from an alginate derivative possessing phenolic hydroxyl moieties (Alg-Ph) via horseradish peroxidase- and catalase-catalyzed reactions. Membrane thicknesses increased from 17 to 40 µm with increasing Alg-Ph concentrations from 0.5 to 2.5% (w/v). In addition, the mechanical strength of microcapsules also increased with increasing Alg-Ph concentrations. Shrinking of microcapsules was observed in Dulbecco’s modified Eagle’s medium and simulated body fluid containing Ca²⁺ (13.1–29.2% reduction in diameter for a 12 d incubation). The diffusivities of dextran (4, 40 or 70 kDa) through a membrane of 2.5% (w/v) Alg-Ph microcapsules were almost the same as those of a 1.5% (w/v) calcium-alginate hydrogel. The results demonstrate that it is possible to tune the characteristics of microcapsule membranes obtained through co-enzymatic reactions to customize the particular applications.