Microwave irradiation is a simple yet effective way of altering the properties of multiwalled carbon nanotubes (MWNTs). This work studies the interactions between microwave-irradiated MWNTs and hydrogen. Effects of MWNT diameter and irradiation duration on the hydrogen-storage capacity have been investigated. We find that microwave irradiation induces damage to the MWNTs that can enhance hydrogen-storage capacity, with excessive damage being detrimental. Smaller-diameter tubes suffer less damage than larger tubes do. MWNTs with a diameter of 20–40 nm irradiated for 10 min had the highest hydrogen uptake of the samples measured, of 0.87 wt% at room temperature and under a hydrogen pressure of 3 MPa. Neutron powder-diffraction data revealed structural changes that were consistent with the insertion of hydrogen in the interstitial cavities of the microwave-irradiated MWNTs, as well as an expansion between the graphene layers of samples that were microwave irradiated. Hence, this simple treatment could be a promising solution to improve the hydrogen-storage capacities of MWNTs.