We use high time cadence images acquired by the STEREO EUVI and COR instruments to study the evolution of coronal mass ejections (CMEs), from their initiation, through the impulsive acceleration to the propagation phase. For a set of 95 CMEs we derived detailed height, velocity and acceleration profiles and statistically analysed characteristic CME parameters: peak acceleration, peak velocity, acceleration duration, initiation height, height at peak velocity, height at peak acceleration and size of the CME source region. The CME peak accelerations derived range from 20 to 6800 m s^2 and are inversely correlated to the acceleration duration and to the height at peak acceleration. 74% of the events reach their peak acceleration at heights below 0.5 Rsun. CMEs which originate from compact sources low in the corona are more impulsive and reach higher peak accelerations at smaller heights. These findings can be explained by the Lorentz force, which drives the CME accelerations and decreases with height and CME size.
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