The stages of extrudate breakup and rounding to form spheroidal pellets was investigated for MCC/water pastes and some MCC/water/calcium carbonate pastes using the interrupted technique reported by Lau et al. (2014). A new quantitative parameter, named ‘dumb-bellity’, was developed to monitor the formation and disappearance of ‘dumb-bell’ shaped pellets in the early stages of the rounding process. Tests using mixtures of coloured extrudates confirmed that attachment of small fragments (‘fines’) to the waist of pellets was not responsible for the transition from dumb-bell to more spheroidal shapes. The results confirmed the findings of Lau et al., that rounding was the rate-limiting step. Extrudates prepared with up to 20 wt% calcium carbonate, (the carbonate representing a hard, active pharmaceutical ingredient), were subject to the same spheronisation mechanism. The time to spheronise the carbonate-containing pastes was longer, which could be related to the increased deformation resistance of these materials.