A gear change mechanism coupling a pair of shafts employs a planetary gear set comprising a sun gear fast with a first of the shafts, planet gears in a planet carrier, and an annulus or ring gear, the latter being fixed. A selector hub slidable on the other of the shafts has sets of radially spaced dogs to engage with corresponding dogs on the planet carrier and sun gear respectively, so as to couple the shafts at two different ratios depending on the axial position of the selector hub. This results in a more axially compact package than conventional dog clutch gear changers. In an alternative embodiment a reversing mechanism is provided by fixing the planet carrier and engaging the selector hub with the sun gear or annulus depending on its axial position.