We recently reported cationic cyclopolymerization of o-vinylbenzaldehydes initiated by boron trifluoride to generate acid-sensitive poly(o-(α-alkyl)vinylbenzaldehyde). Herein we report preparation of core-shell microcapsules (μCs) using flow-focusing microfluidic techniques with shells composed of poly(o-(α-methyl)vinylbenzaldehyde) (PMVB) that release their payload in response to dilute aqueous acid solution. Release profiles of encapsulated fluorescein isothiocyanate-labeled dextran from μCs are controlled by varying the proton concentration and shell-wall thickness. SEM studies indicate that the system's unique reversible release mechanism involves porosity changes in the shell wall due to microcrack formation.