The majority of building blocks in self-assembled capsules feature high symmetry. Reducing this symmetry inevitably leads to expanded possibilities for isomerism. Here, we report a deep cavitand host with one short and three long walls. Its dimerization to hydrogen-bonded capsules in the presence of suitable guest molecules can lead to two constitutional isomers. A given guest induces the predominant formation of only one isomer. The unexpected selectivity is interpreted in terms of the different hydrogen-bonding patterns of the capsules and their effects on the size, shape, and dynamics of the capsules' spaces.