  A first order structural Jahn-Teller transition at T_{JT} = 650 K has been
recently reported for the quadruple perovskite LaMn7O12 . We have carried out
magnetization and transport measurements below and above T_{JT} in order to
investigate the effect of the transition. Electrical conduction turns out to be
polaronic, changing from non-adiabatic to adiabatic through the transition.
Magnetic behavior can be described by non-interacting Mn3+ ions below T_{JT},
while above T_{JT} it is of questionable interpretation. The effect of thermal
cycling on as grown samples of different purity degree also allowed us to
clarify the intrinsic magnetic response of LaMn7O12 at lower temperatures.
