The effect of Mg2+ cations on the electronic spectra and dynamics and efficiency of hole transport has been determined by means of femtosecond time-resolved transient absorption spectroscopy for DNA hairpins possessing stilbene electron acceptor and donor chromophores. The results are compared with those obtained previously for the same hairpins in the presence of Na+ cations and for one hairpin with no added salt. Quantum yields and rate constants for charge separation are smaller in the presence of Mg2+ than Na+, the largest differences being observed for the hairpins with the largest number of base pairs. Slower charge separation is attributed to minor groove binding by Mg2+, which results in a stiffer duplex structure rather than a change in ground state geometry. Reduction in the Na+ concentration has little effect on either the dynamics or efficiency of hole transport.