2H-MoS2 is a semiconductor with a hexagonal layered structure. Each Mo atom is prismatically coordinated by six S atoms. Our in situ X-ray diffraction results indicate that, upon intercalation, the MoS2 host lattice undergoes a first order phase transition in which the Mo coordination changes from trigonal prismatic to octahedral (1T structure). The driving mechanism for this structural change is discussed in terms of a charge transfer from the lithium to the host and in terms of the respective energy-band diagrams for 2H and 1T polytypes. Intercalation-induced reversals in the relative stability of trigonal-prismatic and octahedral phases may also be expected in other semiconducting hosts.