We demonstrate additive shelling of PbS core quantum dots by CdS using room-temperature colloidal atomic layer deposition. Combining in-depth electron microscopy, absorption spectroscopy and elemental analysis, we present a structural model of the PbS/CdS core/shell quantum dots after each of the successive colloidal ALD cycles. In particular, we find that such core/shell quantum dots are cation-rich and we argue that this nonstoichiometry should be attributed to a Cd-excess at the outer surface. Analyzing various PbS/CdS core/shell QDs, we demonstrate that CdS shells have no effect on the energy of the PbS band-edge transition for PbS core QDs larger than ≈4 nm, a finding we support by effective mass calculations. In spite of this, CdS shell growth generally decreases the photoluminescence quantum yield of PbS/CdS core/shell QDs as compared to PbS core QDs. This suggests that photoluminescence quenching is mainly caused by defects at the PbS/CdS interface.