In Ca2PO4Cl the Eu2+ ion occupies two different crystallographic sites. The main difference between the sites is their size. The vibronic coupling strength of the 4f7 to 4f65d transition on Eu2+ is very sensitive to this difference in size resulting in Huang-Rhys factors of S=3 and S=9 for the two sites. The observation of vibronic structure in combination with 7FJ splitting in the excitation spectrum of the Eu2+ emission makes an accurate determination of the splitting of the 4f6 configuration in the 4f65d excited state possible. This splitting is smaller than the free ion values for Eu3+, which is probably due to a reduction of the spin-orbit coupling parameter and/or to crystal-field splitting of the 7FJ terms. Upon irradiation with low energy X-rays Ca2PO4Cl:Eu2+ acts as a storage phosphor. Analysis of the thermoluminescence suggests a centre-to-centre recombination mechanism.