Several experimental techniques are used for phase identification and microstructure characterization of austempered vermicular cast irons (XRD, SEM, TEM and Mössbauer spectroscopy). Acicular structures were found to be composed by ferrite and austenite with average sizes compatible with those reported for bainitic ferrite in steels. An assessment of the free energy change involved in the austenite→bainite transformation indicated a plate-like nucleation shape for bainite with an average characteristic length close to the observed from statistical length distributions. The kinetics of the transformation was modelled in the Avrami framework. Both the diffusion controlled and the diffusionless growth hypothesis were considered in order to elucidate the mechanism underlying the austempering phase transformation.